Hematopoietic-lymphoid membrane antigens that are related to cell differentiation and development, referred to as chicken fetal antigen (CFA) and chicken adult antigen (CAA) were immunochemically characterized; Mr 220,000; Mr 170,000; Mr 130,000; Mr 99,000; Mr 88,000; Mr 50,000; and Mr 24,000 CFA molecules are detected on embryonic RBC, and Mr 210,000; Mr 130,000; Mr 102,000; Mr 56,000; Mr 48,000; and Mr 43,000 CAA molecules are detected on adult RBC. Limited peptide mapping analyses showed all of the CFA and CAA molecules to be distinct entities. Both the Mr 50,000 CFA and the Mr 43,000 CAA molecules exhibited multiple isomorphic variants when analyzed by 2-dimensional electrophoresis. Analyses involving neuraminidase treatments and limited peptide mapping showed the Mr 50,000 CFA isomorphic variants to be chemically identical with the isoelectric point variations being due to sialic acid differences. In addition to multiple isomorphic variants, the molecular weight and charge differences of which were diminished by neuraminidase treatments, the Mr 43,000 CAA molecules exhibited a doublet pattern suggesting that the polyclonal antisera may be detecting chicken major histocompatibility complex products. Analyses of the Mr 50,000 CFA molecules immunoprecipitated with monoclonal antibody 190-4 confirmed that the monoclonal antibody recognizes a serological subset of the Mr 50,000 CFA molecules but showed that it did not recognize a unique molecularly detectable subset among the 18 isomorphic variants discernable by 2-dimensional electrophoretic analyses. Cocapping analyses with splenic lymphocytes showed CFA and CAA to occur as distinct membrane entities on lymphocytes.