Surface molecules involved in human T cell activation were investigated using a newly developed monoclonal antibody (H47 mAb). H47 antigen (Ag) recognized by H47 mAb was expressed on approximately 10% of resting T cells (mostly CD4-CD8+), 30% of phorbol 12-myristate 13-acetate (PMA)-activated T cells (both CD4+CD8- and CD4-CD8+), and most NK, B cells, and monocytes in the peripheral blood mononuclear cells (PBMC). H47 mAb respectively immunoprecipitated a 100 or 120-kD molecular weight (MW) membrane protein of T cells and monocytes under nonreducing or reducing conditions, suggesting that H47 Ag consists of a single polypeptide that has intramolecular disulfide bonds. H47 mAb significantly enhanced PMA-induced proliferation of PBMC in a monocyte-independent fashion. H47 mAb, however, failed to enhance T cell proliferation induced by anti-CD3 mAb, anti-CD2 mAb, or phytohemagglutinin (PHA). H47 mAb also enhanced PMA-induced interleukin-2 receptor (IL-2R) expression and IL-2 synthesis, but did not induce a change in intracellular free calcium ([Ca2+]i) of T cells. These results suggest that H47 Ag is a new membrane molecule involved in PMA-induced T cell activation.