Colony-forming cells (CFC) and colony-stimulating activity (CSA) in peripheral blood cells have been studied before and repeatedly during treatment of 32 patients with acute myelogenous leukemia. WBC obtained after Isopaque-dextran separation were cultured in vitro by a double-layer agar technique. Before treatment 19 patients out of 32 had CSA and all had CFC; both CSA and CFC were found in 19 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 1,000/mm3. This seems to imply that CFC are more sensitive to chemotherapy than colony-stimulating cells. 14 patients entered remission; all of them had CSA. 16 out of the 18 nonresponders lacked on 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. The conclusion from this study is that the presence of both CFC and CSA in peripheral WBC is a sign of a good prognosis. Absence of CFC and/or CSA in the initial sample indicates that the patient is unlikely to respond to conventional therapy with cytotoxic drugs. The presence or absence of CSA alone has a higher prognostic significance than the presence or absence of CFC.