Purified, detergent-soluble HLA antigens (p44,12) are composed of a glycoprotein of molecular weight 44,000 (p44) and a peptide of molecular weight 12,000 (p12), beta2-microglobulin. Upon digestion with papain, p44,12 is converted to p39,12, then to p34,12, which retains antigenic activity. The NH2-terminal amino acid sequences of p34 and p44 are identical. p44, p39, and p34 were purified, and comparison of their amino acid compositions showed that the COOH-terminal peptide removed by the first papain cleavage is hydrophilic and contains cysteine that can be alkylated after mild reduction. The penultimate COOH-terminal peptide removed by the second papain cleavage is hydrophobic, and presumably anchors HLA antigens to the membrane. This correlates with the observation that p44,12 and p39,12 bind detergent, while p34,12 does not. The orientation and integration of HLA antigens in the lymphocyte membrane were thus defined, and the structure suggests that HLA antigens span the plasma membrane.