Granulocyte-macrophage progenitor cells (CFU-GM), leukocyte colony-stimulating activity (CSA), granuloblast differentiation and proliferation and the effect of uremic serum on the in vitro growth of normal CFU-GM have been studied in 8 chronic uremic patients treated with intermittent hemodialysis three times a week. The studies were performed in the postabsorptive state twice in each patient, that is at the longest and shortest dialytic interval. CFU-GM growth in agar and leukocyte CSA did not differ significantly from the normal level in uremic subjects. The granulocytic and macrophagic differentiation in a liquid culture system was significantly reduced in uremic patients, notwithstanding the appearance of high numbers of undifferentiated blastic cells. The serum of uremic patients had no effect on normal CFU-GM and leukocyte CSA.