The chemistry of antigens controlled by the major histocompatibility complex of the chicken has been investigated. Peripheral blood leukocytes of five chicken strains were radioactively labeled by incorporation of tritiated amino acids, enzymatic iodination, or mild periodate oxidation followed by reduction with tritiated sodium borohydride. Membrane-bound antigens were solubilized with Nonidet P-40 and purified by a two-step immunoprecipitation procedure. The resulting immunoprecipitates were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under reducing or nonreducing conditions. The chicken major histocompatibility antigens (B antigens) are composed of polypeptide chains with molecular weights (in the reduced form) of 40,000 to 43,000 and 11,000 to 12,000, as deduced from their mobilities in internally calibrated gels. The larger chains, when isolated from different chicken strains, can have slightly different mobilities in such gels; thus the mobilities of the large polypeptides from strains WA and WB correspond to molecular weights of 40,000 and 43,000, respectively. Gels run under nonreducing conditions give similar results; in particular no dimers or oligomers of the larger chains are detectable, thus ruling out an immunoglobulin-like structure for the B antigens. The large chains are labeled by the periodate oxidation-reduction procedure, suggesting that they are glycoproteins (probably containing sialic acid), while the small polypeptides cannot be labeled by this method. An additional protein or proteins of apparent molecular weight about 30,000 could be precipitated from 3H-amino-acid-labeled leukocyte lysates by one particular anti-B serum. This protein(s), named B-L, differs from the B major histocompatibility antigens in that it is not associated with the small chain of molecular weight 11,000 to 12,000 and does not occur on erythrocytes. Thus, at least two protein chains are coded for (or have their synthesis controlled by) genes in the chicken major histocompatibility complex.