Eight adult patients suffering from leukocytopenia have been injected by droplet intravenous infusions with partially purified human urinary colony-stimulating factor (CSFHU), which stimulated human monocytes to produce colony-stimulating activity (CSA) in vitro. Three of eight patients were injected with low-dose CSFHU and five with high-dose CSFHU. The infusion of high-dose CSFHU led to a slightly earlier rise in absolute neutrophil counts and a higher CSA level in the serum, as compared to noninjected leukocytopenic patients. There was little toxicity associated with it. Human urinary colony-stimulating factor used in the clinical studies did not have any CSA in vitro in the presence or absence of the patients' sera. These data may suggest that the intravenous infusion of CSFHU increased the serum CSA level, probably through the stimulation of CSA-producing cells in vivo.