A monoclonal antibody (ISU-cA) was characterized that recognized certain alloantigens of the chicken A blood group locus. It was specific for A[superscript]3, A[superscript]4 and A[superscript]8 antigens, but not for A[superscript]2 or A[superscript]5 antigens. Specificity of ISU-cA for A blood group antigens was demonstrated by serologic analyses, genetic crosses, competitive binding between ISU-cA and blood group A-specific alloantisera and immunoprecipitation of identical molecules with ISU-cA and alloantisera. Blood group A antigens were detectable on erythrocytes from 3 days of incubation and gradually increased in expression until adulthood. Antigen expression of heterozygotes was intermediate in level between those of each homozygous parent confirming the co-dominant expression of blood group antigens. Flow cytometric analyses showed that the increase in antigen expression with age was due to an increase in the level of A antigen expression on individual erythrocytes. The SDS-PAGE showed relative molecular weights of 53 Kd for A[superscript]2 and 54.5 Kd for A[superscript]4. The non-reduced molecular weight for both antigens of 44.5 Kd suggested the presence of intra-chain disulfide bonds. Two-dimensional gel electrophoresis indicated that A blood group antigens consist of a single, probably glycosylated, molecule with pI of approximately 6.5.