Many familiar abbreviations, capitalized and written without periods, are acceptable in formal writing.
Initialisms are abbreviations formed from the first letters of words (MLB, HTTP); acronyms are initialisms that can be pronounced as words (NATO, AIDS).
Organizations
NATO, CIA, FBI, AFL-CIO, NAACP, IBM, UPI, MLA, APA
Technical terms
CD-ROM, OCR, WORM, XHTML, LED, PDF
Scientific terms
MMRI, HPLC, MCSGP, ATP, NADH
Television and radio stations
CBS, MSNBC, KTWO, WABC-TV
Miscellaneous terms
IOU, ESL
Style note: Acronyms and initialisms
Exercises:
Abbreviations 1
Abbreviations 2
Related topic:
Capitalizing abbreviations