We recently identified a group of proteins that are present in all hematopoietic cells but are organized in the cell membrane of erythrocytes in a lineage-specific fashion (Blood 61:803, 1983). One of these polypeptides has a mol wt of approximately 37,000 (p37T) when translated in vitro from messenger RNA (mRNA) extracted from the erythroleukemic K562 cell line. The membrane-associated form of the p37 translation product has been analyzed in detail here. When detergent lysates prepared from biosynthetically labeled K562 cells were reacted with an antiserum containing anti-p37T antibodies, one of the proteins immunoprecipitated had a nominal mol wt of 36,000 to 37,000 (p37M). Several results suggest that this protein is homologous to the p37 translation product: (1) the protein, like the mRNA coding for the p37 translation product, was expressed in cell lines with diverse differentiated phenotypes; (2) the antigenic determinant(s) on p37M and p37T are oriented to the inner surface of the erythrocyte membrane while being oriented to the outer surface of erythroleukemic cells; and (3) one-dimensional peptide maps show homology between p37M and p37T. P37M does not appear to possess an N-terminal leader sequence that is proteolytically cleaved as the molecule is inserted into the membrane. In addition, p37M is not glycosylated.