Comparison of NH2-terminal protein sequence from the rat OX-44 antigen with the sequence of the human CD37 antigen deduced from a cDNA clone shows that these antigens are species homologues. The CD37 sequence is 244 amino acids in length and lacks a conventional leader sequence. The molecule is likely to have an NH2-terminal cytoplasmic domain followed by three transmembrane sequences that lie within the first 110 amino acids. The rest of the molecule is hydrophillic and contains three sites for N-linked glycosylation.