A new serum-free assay system for leukemic colony formation (leukemic colony-forming units, L-CFU) was established, and, utilizing this system, the colony-promoting activities of recombinant human colony-stimulating factors (rhCSFs) and the effectiveness of CSFs on cellular self-renewal capacity were investigated. The serum-free assay system included deionized bovine serum albumin (1%), cholesterol (7.8 micrograms/ml), and ASF 101 medium. The plating efficiencies obtained by culturing with phytohemagglutinin-stimulated leukocyte-conditioned medium (PHA-LCM) ranged from 0.01% to 1.35% in this system. Spontaneous colonies were observed in 9 out of 13 cases studied. Recombinant human granulocyte CSF (rhG-CSF), rh granulocyte-macrophage CSF (rhGM-CSF), rh interleukin 3 (rhIL-3), and rh interleukin 1 (rhIL-1) stimulated colony formation in 10, 9, and 6 out of 13, and 5 out of 11 cases, respectively. The magnitude of stimulation by each CSF ranged from 6% to 145%, 21% to 200%, 0% to 1229%, and 14% to 182% of PHA-LCM, respectively. In eight cases, blast colony assays in serum-free and serum-containing cultures were simultaneously performed. The magnitudes of responsiveness of each CSF differed in the two assay systems; this indicated some effect of fetal calf serum. The value of self-renewal capacity was also examined and compared with that in serum-containing culture. Self-renewal capacity could be maintained with a serum-free culture system. It showed marked variation from case to case, and there was no correlation between the primary colony formation and the self-renewal capacity in both culture systems. Taken together, the development of a completely serum-free culture system was found to be efficient as evaluated by a L-CFU colony assay.