The monoclonal antibody WR16 was secreted by a hybridoma produced by fusing splenocytes from a BALB/c mouse immunized with human T-cell chronic lymphocytic leukaemia cells and the murine myeloma cell line NS-O. WR16 reacts specifically with human lymphocytes and binds to 48% of OKT4+ T lymphocytes and the majority of B lymphocytes. Human OKT4+ tonsil lymphocytes were subfractionated into WR16-/OKT4+ and WR16+/OKT4+ subpopulations by the panning technique. The capacity of these cells to help or suppress pokeweed mitogen-induced immunoglobulin (Ig) secretion by autologous B lymphocytes was monitored after 9 days of coculture. Cells of phenotype WR16-/OKT4+ enhanced Ig secretion in excess of that found with non-fractionated OKT4+ lymphocytes, whilst WR16+/OKT4+ lymphocytes suppressed Ig secretion when added to a mixture of B lymphocytes and non-fractionated OKT4+ cells. The WR16-/OKT4+ subpopulation was further fractionated by use of the MoAb Leu 8. Forty-two percent of WR16-/OKT4+ lymphocytes bound Leu 8, and cells of the phenotype WR16-/OKT4+/Leu 8+ were found to induce B-lymphocyte Ig secretion, whilst WR16-/OKT4+/Leu 8- lymphocytes were less active in this system. These data confirm the heterogeneity of the human T helper/inducer subset and indicate the existence of a population of OKT4+ lymphocytes that can suppress Ig secretion in the absence of OKT8+ lymphocytes.