A rabbit antiserum against commercially available Electrophorus electricus acetylcholinesterase has been prepared. Five precipitation bands were distinguished by immunoelectrophoresis, but only three of these contained demonstrable enzyme activity. In an in vitro system, activity of the commercial enzyme or of highly purified acetylcholinesterase was inhibited by 70-82 per cent after incubation with antiserum. Antibody specificity was demonstrated by the absence of serological cross reactions or enzyme inhibition with bovine erythrocyte acetylcholinesterase or horse serum cholinesterase.