A basophil-like cell promoting activity (BaPA) is a lymphokine which has the ability to proliferate metachromatically staining cells in human bone marrow. We studied the effect of BaPA on histamine release from human peripheral basophils. BaPA did not directly induce histamine release from basophils. However BaPA ranging from 0.001 to 0.5 U/ml enhanced histamine release from basophils stimulated with anti-IgE, calcium ionophore A23187 or FMLP in a dose-dependent manner. On the basis of recent observations that in addition to their capacity for proliferating progenitor cells, colony-stimulating factors are capable of regulating functions of end-stage cells of the same lineage, this finding that BaPA is able to modulate a basophil function raises the possibility that BaPA also regulates progenitors of basophils.