SummaryExpression of chicken red blood cell (RBC) surface antigens was studied by using a monoclonal antibody (ISU‐cA) specific for chicken A blood group antigens. Erythrocytes were examined from embryos of 3–18 days of incubation and from chicks at hatch up to 21 weeks of age. Specific antigens were detected on embryonic RBC surfaces by immunofluorescence as early as 3 days of incubation. Antigenic expression was examined by both haemagglutination and immunofluorescence and found to increase with age from embryos to mature birds. The antigen concentration on the cell surface was found to be affected by genotype; heterozygotes had an intermediate level of antigen between that of the two parental genotypes. These data confirm the co‐dominance that is observed with most blood group antigens. Flow cytometric analysis allowed confirmation that the entire erythrocyte population gradually increased in antigenic expression over time, rather than having an antigen‐negative subpopulation being replaced by a positive subpop