CD45 leucocyte common antigen (LCA)belongs to the family of at least four isoforms of membrane glycoproteins (220, 205, 190, 180kDa) expressed on hematopoietic cell lines but absent on non-hematopoietic cell lines, normal and malignant non-hematopoietic tissues. The intracellular portion of these molecules have protein phosphatase activity and are involved in regulation of transmembrane signals. Antibody to CD45 is useful in differential identification of lymphoid tumors from non-hematopoietic undifferentiated neoplasms.