To evaluate the effects of recombinant G-CSF and GM-CSF on RAEB and RAEB-T cells, blast cells from 6 patients were incubated in liquid culture systems with these CSFs for 7 days, and their numerical, morphological and functional changes were assessed. Both CSFs stimulated cell growth, but decreased the proportion of blast cells in 5 of the 6 cases. Karyotypic abnormalities persisted during cultivation in some cases. The CSFs also stimulated the expression of part of the esterase activities, and a positive interaction of both CSFs was seen in part. Although CSFs had no significant effects on the ability of cells to reduce NBT or to phagocytize latex particles, the results indicated that they induce partial differentiation of blast cells. It appears that such pathological cells still retain the capacity to respond to growth factors.