Papain-solubilized HLA-A,B,C antigen heavy chains have been cleaved by combined acid and CNBr treatment to yield three large fragments. A 14,000-dalton peptide comprises the NH2-terminal portion of the molecule, less a five-membered peptide. The 14,000-dalton fragment is followed in the linear sequence by a 9000-dalton peptide connected through an aspartyl-prolyl bond to the COOH-terminal 11,000-dalton fragment. The 9000- and 11,000-dalton fragments contain disulphide bridges that are immunoglobulin-like inasmuch as they encompass some fifty-five to sixty amino acid residues. The NH2-terminal portion of the HLA antigen heavy chain is devoid of cysteine. NH2-terminal amino acid sequence analyses do not reveal homologies between the 14,000- and 9000-dalton fragments, beta2-microglobulin, and the constant immunoglobulin domains. However, the NH2-terminal sequence of the 11,000-dalton fragment is as homologous to beta2-microglobulin and the constant immunoglobulin domains as they are to one another.