| MAC | Market Access and Compliance |
| MAPS | Mapping and Analysis for Public Safety |
| MARAD | Maritime Administration |
| MARVEL | Machine Assisted Realization of the Virtual Electronic Library |
| MATRIX | Multi-State Anti-Terrorism Information Exchange (.pdf) |
| MBCC | Migratory Bird Conservation Commission |
| MBDA | Minority Business Development Agency |
| MCC | Millennium Challenge Corporation |
| MCHB | Maternal and Child Health Bureau |
| MCTF | Middle Class Task Force |
| MCTL | Militarily Critical Technologies List |
| MDA | Missile Defense Agency |
| MDB | Multilateral Development Banks |
| MDW | Military District of Washington |
| MedPAC | Medicare Payment Advisory Commission |
| MEP | Manufacturing Extension Partnership |
| MEPS | Medical Expenditure Panel Survey |
| MeSH | Medical Subject Headings |
| MIGA | Multilateral Investment Guarantee Agency |
| MLS | Mass Layoff Statistics |
| MMC | Marine Mammal Commission |
| MMS | Minerals Management Service |
| MMWR | Morbidity and Mortality Weekly Report |
| MOLIS | Minority On-Line Information Service (defunct) |
| MPSA | Military Postal Service Agency |
| MRC | Medical Reserve Corps | Mississippi River Commission |
| MRM | Minerals Revenue Management |
| MSA | Metropolitan Statistical Area |
| MSC | Military Sealift Command |
| MSDS | Material Safety Data Sheets |
| MSEL | Materials Science and Engineering Library |
| MSHA | Mine Safety and Health Administration |
| MSPB | Merit Systems Protection Board |
| MTDP | Minority Telecommunications Development Program |
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