SummaryA monoclonal antibody (ISU‐cA) was produced that recognized certain alloantigens of the chickenAblood group locus. Antigens produced by allelesA3,A4andA8were positive, and those produced byA2andA5were negative, by haemagglutination. The specificity of ISU‐cA for chicken A blood group antigens was demonstrated by serologic analyses, genetic crosses and competitive inhibition of binding by anti‐A alloantisera. To our knowledge, this is the first reported monoclonal antibody against a chicken blood group alloantigen system other than theBco