To clarify the role of T lymphocytes or monocytes in the regulation of human peripheral blood erythroid burst forming unit proliferation, equal numbers of purified T cells, B cells or monocytes were cultured in vitro with null cells in the plasma clot system. BFUE proliferation was directly proportional to the T cell concentration and inversely proportional to monocyte concentration in culture. The effect of B cells was variable and dependent on the extent of T cell contamination. Addition of a highly specific anti-human thymocyte IgG preparation (ATG) to null cells, T cells + null cells or B cells + null cells significantly reduced or eliminated BFUE proliferation. A burst promoting activity (BPA) was found in the supernatants of T cell enriched mixed lymphocyte cultures and could oppose the inhibition of BFUE proliferation by ATG. A burst inhibitory activity was found in the supernatants of monocyte enriched cultures. These studies provide further evidence that BFUE proliferation is determined by opposing actions of stimulator T cells and inhibitor monocytes and/or their soluble products.