The regulation of erythroid burst-colony formation was studied in cultures of human peripheral blood mononuclear cells. Numbers of erythropoietin-stimulated colonies obtainable from the cells in response to various treatments were compared. One-day preincubation of the cells with phytohemagglutinin (PHA) doubled the yield of colonies. Irradiation of the cells with 3000 rad eliminated their ability to form erythroid bursts, but did not impair the ability of PHA-treated cells to enhance burst formation when added to a fresh batch of cells. This was due to a humoral factor, since media conditioned by PHA-treated washed cells were as effective as the cells themselves. When cells were separated into subpopulations by an adherence procedure and according to their ability to form rosettes with sheep red blood cells, it was found that the PHA-dependent burst-promoting activity released into the medium originated in a nonadherent, nonrosetting (T-cell depleted) cell population.