Assays were performed on bone marrow colony-stimulating-factor (CSF) levels in the serum and urine of 33 patients with acute granulocytic and myelomonocytic leukemia. Of 251 serum samples, 30% showed elevated CSF levels as did 53% of 1422 urines tested. Levels of serum inhibitors of colony formation were abnormally low or undetectable in 57% of sera from leukemic patients. Patients in relapse were unable to respond to infections by developing high serum CSF levels and low serum-inhibitor levels were more common in patients with a short survival time and in some types of patient during relapses.