Glycoprotein CD45, a transmembrane protein tyrosine phosphatase of leukocytes, is topographically similar to the epidermal growth factor receptor, a transmembrane tyrosine kinase. Since the latter is thought to be allosterically regulated through conversion between monomeric and dimeric forms, we sought to determine whether CD45 undergoes similar oligomerization. Our analysis, employing a thiol-cleavable and homobifunctional chemical cross-linker, dithiobis succinimidyl propionate, revealed that CD45 indeed formed homodimers. In addition, a protein of molecular mass 30,000 daltons (30 kDa) was found to be associated with both the CD45 monomer and dimer. The 30-kDa protein was phosphorylated and was not labeled by cell surface radioiodination. Distinct differences in protein tyrosine phosphatase activity were detected among the various populations of CD45 separated by sucrose gradient ultracentrifugation. However, the differences observed could not be explained simply by dimerization and instead suggest the presence of other factor(s) involved in the regulation of CD45 enzyme activity.