The effects of interleukins 3 and 6, stem cell factor, and granulocyte-macrophage colony-stimulating factor on human fetal hematopoietic, bone marrow, and cord blood cells were studied on the basis of the colony-forming capacity. Fetal hematopoietic cells from 28 elective abortions, three bone marrow samples, and three cord blond samples were incubated with cytokines and investigated for the presence of BFU-E (burstforming units – erythroid), CFU-GM (colony-forming units – granulocytes, macrophages), and CFU-GEMM (colony-forming units – granulocytes, erythrocytes, macrophages, megakaryocytes). Single and combined cytokines and preincubation versus adding cytokines in culture were investigated. Interleukin-6 alone had the most pronounced effect on BFU-E formation. All four cytokines in combination yielded the highest scores for CFU-GM (p < 0.05) and CFU-GEMM (p < 0.05), whereas BFU-E was not enhanced. The mode of cytokine exposure was not a determinant of colony form