Mammoth Uncensored Word List:
12 words that are 21 letters long beginning with a in the mammoth uncensored word list.

acetylcholinesterases acrocephalosyndactyly adrenocorticosteroids adrenocorticotrophins anatomicophysiologies anatomicophysiologist anthropomorphizations antiauthoritarianisms antienvironmentalists antiferromagnetically antiinstitutionalists astrophotographically