The leukocyte common antigen, CD45, is one of the major glycoproteins on cells of hemopoietic origin showing considerable heterogeneity in both structure and expression. Biochemical heterogeneity has been attributed to differences in the primary sequence and glycosylation. In this paper we report an additional basis for generation of heterogeneity by revealing that CD45 can form disulfide-bound heterodimers with an 80 kDa polypeptide. Since a respectable fraction of the CD45 molecules is involved in heterodimer formation, it is suggested that the 80 kDa polypeptide could be involved in the regulation of CD45 function.