| 501 | MAOG | MANGUM OIL & GAS INC | | | SCAC4 |
| 502 | MAOI | MAGNO INTERNATIONAL LP | | | SCAC4 |
| 503 | MAOL | MAY OIL COM | | | SCAC4 |
| 504 | MAON | MITS AIRCONDITIONING INC | | | SCAC4 |
| 505 | MAOP | MAR-CO CLAY PRODUCTS LTD | | | SCAC4 |
| 506 | MAOR | MARIOS AIR | | | SCAC4 |
| 507 | MAOT | MARTIN BROTHERS TRUCKING INC | | | SCAC4 |
| 508 | MAOV | MID-AMERICA OVERSEAS | | | SCAC1 |
| 509 | MAOV | MID AMERICA OVERSEAS INC | | | SCAC4 |
| 510 | MAOW | MT ALBERTA | | | SCAC4 |
| 511 | MAOY | MARTIN OIL CO INC | | | SCAC4 |
| 512 | MAP | National Oceanic & Atmospheric Administration | Callsign: Mapping, | USA | AVCODES |
| 513 | MAP | Altair Jets, C.A. | Callsign: Altjets, | YV | AVCODES |
| 514 | MAPA | MARYLAND & PENNSYLVAINA | | | PWRC |
| 515 | MAPC | M & A EXPRESS INC | | | SCAC4 |
| 516 | MAPE | MADARIS TRANSPORTATION LLC (PIERSID | | | SCAC4 |
| 517 | MAPF | MAPES FUEL OIL INC | | | SCAC4 |
| 518 | MAPI | MASTRONARDI PRODUCE (1995) INC | | | SCAC4 |
| 519 | MAPK | MARX TRUCKING | | | SCAC4 |
| 520 | MAPLE | Maplewood Equipment Co. | | USA | TRAFFIC |
| 521 | MAPN | MID-ATLANTIC TRANSPORT INC | | | SCAC4 |
| 522 | MAPQ | MAP LOGISTICS INC | | | SCAC4 |
| 523 | MAPU | MAP LINE INDIA PRIVATE LIMITED | | IND | BIC |
| 524 | MAPV | MAP TRANSPORTATION INC | | | SCAC4 |
| 525 | MAPW | MONTINA CORPORATION | | | SCAC4 |
| 526 | MAPX | Semgroup LP | added 4/2005; 7/2005 | | NAKINA |
| 527 | MAPX | Williams Energy Ventures, Inc. | added 10/1999; 7/2000; eliminated 4/2005 | | NAKINA |
| 528 | MAPX | Mapco Petroleum, Inc. | added 10/1993; 1/1994; eliminated 10/1999 | | NAKINA |
| 529 | MAPX | Mapco Gas Products, Inc. | eliminated 10/1993 | | NAKINA |
| 530 | MAPX | Mapco Petroleum, Inc. | added 4/1985 | | NAKINA |
| 531 | MAPX | Mapco Products Co. | added 10/1980; eliminated 4/1985 | | NAKINA |
| 532 | MAPX | Mapco Products Co. | added 10/80; eliminated 4/85 | | NRHS |
| 533 | MAPX | Mapco Petroleum, Inc. | added 4/85 | | NRHS |
| 534 | MAPX | Mapco Gas Products, Inc. | eliminated 10/93 | | NRHS |
| 535 | MAPX | Mapco Petroleum, Inc. | added 10/93; 1/94; eliminated 10/99 | | NRHS |
| 536 | MAPX | Williams Energy Ventures, Inc. | added 10/99; 7/2000 | | NRHS |
| 537 | MAPX | SEMGROUP LP [WILLIAMS ENERGY VENTURES; MAPCO PETROLEUM; MAPCO GAS PRODUCTS; MAPCO PRODUCTS CO.] | | | PWRC |
| 538 | MAPX | MAPCO Products | | | RSRV |
| 539 | MAPX | SEMGROUP LP | | | SCAC4 |
| 540 | MAQ | MAC Aviation, S.A. | Callsign: Mac Aviation, | E | AVCODES |
| 541 | MAQ | Mac Aviation | | | ICAO1 |
| 542 | MAQB | MID-ATLANTIC BULK CARRIERS LTD | | | SCAC4 |
| 543 | MAQD | MAQUILADOS PDT S A DE C V | | | SCAC4 |
| 544 | MAQF | MAYA FREIGHT SYSTEMS | | | SCAC4 |
| 545 | MAQK | MARTELLIS INC | | | SCAC4 |
| 546 | MAQR | MALSON TRANSPORTATION INC | | | SCAC4 |
| 547 | MAQT | MAQTEC S A DE C V | | | SCAC4 |
| 548 | MAQW | MANNLINES | | | SCAC4 |
| 549 | MAR | March Helicopters Ltd. | Callsign: March, | GB | AVCODES |
| 550 | MAR | Air Mediterranean S.A. | IATA: MV, Callsign: Hellasmed, IATA Prefix: 012, Commenced operations on 01 November 2017. Suspended all operations on 18 January 2018. Re-started operations in February 2018 operating charter flights only. | GR | AVCODES |
| 551 | MAR | Air Mediterranean S.A. | IATA: MV, Callsign: Hellasmed, Commenced operations on 01 November 2017. Suspended all operations on 18 January 2018. Re-started operations in February 2018 operating charter flights only. | GR | AVCODES |
| 552 | MAR | March Helicopters | | | ICAO1 |
| 553 | MAR | Missouri and Arkansas Ry. Co. | 7/1935-4/1938 | | NAKINA |
| 554 | MAR | Missouri and Arkansas Ry. Co. | Apr-38 | | NRHS |
| 555 | MAR | *MISSOURI & ARKANSAS RWY. CO. | | | PWRC |
| 556 | MAR | *MARTRAC | | | PWRC |
| 557 | MAR | Trafikverket Materialservice Rälsverkstaden | | SE | VKM |
| 558 | MARB | MARIANO BROTHERS, INCORPORATED | | | SCAC4 |
| 559 | MARB | Marathon Bus Service | | US-NJ | TRAFFIC |
| 560 | MARC | Maryland Transit Administration | 7/2005 (not listed) | | NAKINA |
| 561 | MARC | *MARQUETTE CEMENT | | | PWRC |
| 562 | MARC | +MARYLAND TRANSIT ADMINISTRATION | | | PWRC |
| 563 | MARC | MARK IV TRUCKING CO | | | SCAC4 |
| 564 | MARC | Maryland Rail Commuter | | USA | TRAFFIC |
| 565 | MARCIN | Marietta & Cincinnati Railroad | | USA | TRAFFIC |
| 566 | MARD | MARINER DISTRIBUTION INC | | | SCAC4 |
| 567 | MARG | MAR TRANSPORT | | | SCAC4 |
| 568 | MARI | MARELI INTERNATIONAL INC | | | SCAC4 |
| 569 | MARIN | Marin Airporter | | US-CA | TRAFFIC |
| 570 | MARK | MARK TRANS LTD | | | SCAC4 |
| 571 | MARKAB | Zgierskie Przedsiebiorstwo Komunikacyjne "MARKAB" Sp. z o.o. | | PL | TRAFFIC |
| 572 | MARKLIN | Märklin | | X | TRAFFIC |
| 573 | MARL | MARCHAND CONSTRUCTION INC | | | SCAC4 |
| 574 | MARM | Ferrovia Marmifera Privata di Carrara | | I | TRAFFIC |
| 575 | MARN | MARS TRUCKING INC | | | SCAC4 |
| 576 | MARQ | M.O. AIR INTERNATIONAL, INC | | | SCAC1 |
| 577 | MARS | MARS SERVICE COMPANY INC | | | SCAC4 |
| 578 | MART | American Refrigerator Transit Co. | added 4/1960; 4/1961-10/1966; eliminated 10/1969 | | NAKINA |
| 579 | MART | American Refrigerator Transit Co. | 4/61-10/66; eliminated 10/69 | | NRHS |
| 580 | MART | AMERICAN REFRIGERATOR TRANSIT CO. | | | PWRC |
| 581 | MART | Martinair | | | TRAFFIC |
| 582 | MARTA | METROPOLITAN ATLANTA RAPID TRANSIT AUTHORITY | | | PWRC |
| 583 | MARTA | Metropolitation Atlanta Rapid Transit Authority | | USA | TRAFFIC |
| 584 | MARTI | Marti Infra AG | | CH | TRAFFIC |
| 585 | MARTI | Marti Infra AG | | CH | VKM |
| 586 | MARU | CARU CONTAINERS BV | | NL | BIC |
| 587 | MARUB | MARUB SA | | RO | VKM |
| 588 | MARUG | Marugame Community Bus | City: Marugame | J | TRAFFIC |
| 589 | MARW | MAUS, R W TRANSPORT INC | | | SCAC4 |
| 590 | MARW | MARWAY FERTILCHIM SA | | RO | VKM |
| 591 | MARX | MidAmerican Energy Co. | added 10/1998; 7/2000-7/2005 | | NAKINA |
| 592 | MARX | North American Car Corp. | added 7/1955; 10/1957-4/1970; eliminated 7/1973 | | NAKINA |
| 593 | MARX | Mather Stock Car Co. | added 10/1953; 7/1954; eliminated 7/1955 | | NAKINA |
| 594 | MARX | Marland Tank Line Co. (Marland Refining Co.) | 7/1923 | | NAKINA |
| 595 | MARX | Marland Refining Co. | 8/1920 | | NAKINA |
| 596 | MARX | Mather Stock Car Co. | added 10/53; eliminated 7/55 | | NRHS |
| 597 | MARX | North American Car Corp. | added 7/55; 10/57-4/70; eliminated 7/73 | | NRHS |
| 598 | MARX | MidAmerican Energy Co. | added 10/98; 7/2000 | | NRHS |
| 599 | MARX | *MARLAND TANK LINE (MARLAND REFINING CO.) | | | PWRC |
| 600 | MARX | *ARMOUR & CO. (NCCO [MATHER STOCK CAR CO.]) | | | PWRC |
