The amino acid sequence of the cytosolic human placenta protein-tyrosine-phosphatase 1B (PTPase 1B; protein-tyrosine-phosphate phosphohydrolase, EC 3.1.3.48) has been determined. It consists of a single chain of 321 residues with an N-acetylated N-terminal methionine and an unusually proline-rich C-terminal region. The enzyme is structurally related to the two cytoplasmic domains of both the leukocyte common antigen CD45 and LAR, a CD45-like molecule with an external segment that resembles a neural cell adhesion molecule. A low molecular weight protein encoded by a cDNA clone from T cells also shows extensive sequence similarities. The present study defines homologous domains common to this diverse family of PTPases that includes both soluble and receptor-like transmembrane forms. The cysteinyl residues 121 and 215 of PTPase 1B are conserved among all members of the family and are candidates for involvement in catalysis since PTPase 1B is inactivated by thiol modifying reagents. Two segments rich in positively charged residues (residues 33-47 and 227-238) may provide sites of interaction with inhibitory anionic polymers such as heparin or poly(Glu/Tyr).