The CD3 epitope is expressed on the epsilon chain of the CD3/T cell antigen receptor (TcR) complex. CD3 is present on 65-85% of thymocytes and has a mitogenic effect on peripheral blood T cells. Indentification of human T cells expressing the 22-28,000 M.W. surface antigen. CD44 is a 90kD (GP90), collagen-binding, membrane glycoprotein, termed extracellular matrix receptor III (ECMR III), that is homologous to the lymphocyte homing receptor and CD44 antigen. CD44 is abundantly expressed in many epithelial tissues, and is localized predominantly to filopodia in cultured keratinocytes. CD44 is a polymorphic family of related membrane proteoglycans and glycoproteins possessing extensive diversity in both glycosylation and core protein sequence. Human neonatal foreskin keratinocytes (HFKs) and QG56 lung squamous carcinoma cells express an alternatively spliced form of the CD44 core protein (termed CD44E) that contains an additional 132 amino acids in the carbohydrate attachment region of the extracellular domain. HFKs, HT1080 fibrosarcoma and QG56 cells, as well as many other human cells, contain varying ratios of GP90. Differences in mass are due primarily to variation in carbohydrate moieties, including sulfated aspargine-linked glycopeptides (GP), chondroitin sulfate (CS), and heparan sulfate (HS) glycosaminoglycans, as well as O-linked mucin and polylactosamine structure(s).