Colony-forming cells (CFC) and colony-stimulating activity (CSA) in peripheral blood cells have been studied before and repeatedly during treatment of 30 patients with acute non-lymphoblastic leukemia. WBC obtained after Isopaque-dextran separation were cultured in vitro by a double-layer agar technique. Before treatment 16 patients out of 30 had CSA and 22 out of 29 had CFC; both CSA and CFC were found in 15 patients. In follow-up studies during treatment, CSA was mainly unaffected during the leukopenic phase, while CFC were suppressed. No CFC were found at WBC counts below 900/mm3. This seems to imply that CFC are more sensitive to cytotoxic agents than colony-stimulating cells. Twelve patients entered remission; all of them had CSA and all the 11 who were investigated for CFC had CFC before treatment. Fourteen out of 18 non-responders lacked one or both types of cells. The presence of CSA and CFC in peripheral blood therefore appears to be a sign of favorable prognosis, while the absence of CSA and/or CFC implies lack of response to treatment.