HLA-A2 and -B7 antigens are phosphorylated by Rous sarcoma kinase (pp60v-src) in vitro. The phosphate group is attached to the heavy chains as determined by NaDodSO4/polyacrylamide gel electrophoresis. The site of phosphorylation was localized to the COOH-terminal intracellular domain by its susceptibility to limited trypsin proteolysis. Furthermore, the 32P-labeled amino acid is a single tyrosine residue located in the COOH terminus of the heavy chain. The protein sequences of known class I human and murine intracellular domains contain a highly conserved sequence -K-G-G-X-Y- located NH2-terminally to the single tyrosine residue of this domain. The DNA sequences that encode class I antigen intracellular domains were compared by computer with a homology matrix program. Exon 6 which encodes the conserved tyrosine-containing protein sequence in both human and mouse is 75% homologous across species and 90-100% homologous within species. The significance of the high degree of conservation within exon 6 is discussed.