A monoclonal antibody (MAb), CH11, was developed by immunizing mice with CD4+ γ δ T-cell receptor (TCR)+ cells. It recognized an antigen expressed in the surface membrane of T-cell lines, but not of U937, lymphoblastoid B cells (LBC), K562, Raji or Daudi cells, indicating selectivity for the T-cell lineage. In addition, it labelled 70-80% of normal peripheral blood mononuclear cells (PBMC), with high expression on the erythrocyte rosetting (E+) fraction, and low/absent expression on E- cells. However, CD4+ T cells expressed higher levels of reactivity than CD8+ or γ δ+ T-cell receptor (TCR)+ lymphocytes in PB. Furthermore, in 7 of 10 individuals tested, 7.34 ± 63.88% of unselected PBMC were CH11- CD3+ and were relatively enriched in CD8+ and in γ δ TCR+ -cells. In addition, thymic γ δ T cells, and γ δ lymphoproliferations from two patients were nonreactive or weakly reactive with the MAb. Activation of E+ cells with phorbol-12-myristate-13-acetate (PMA) enhanced CH11 expression uniformly, whereas activation with phytohemagglutinin (PHA) selectively down-regulated expression of the antigen on the CD8+ subset. In Western blots performed in nonreducing (NR) conditions, MAb CH11 detected a 100 kDa molecule in PBMC and Jurkat T-cell lysates. Preincubation of T cells with MAb CH11 specifically abrogated their subsequent reactivity with MAb to CD6, suggesting that MAb CH11 is recognizing an epitope of CD6. Given its function as a receptor for ligands on thymic epithelium, activated leukocytes and synoviocytes, this newly defined heterogeneity of expression and regulation of the CD6 molecule on subsets of T cells may help determine their functional repertoirein vivo.