Human monocytes release cytostatic protein factors (CF) upon activation with lymphokines and lipopolysaccharide. CF has been purified from monocyte supernatants by ion-exchange chromatography, chromatofocusing and gel filtration, and this resulted in a 10,000-fold reduction in the amount of protein in the purified CF preparation compared to the amount in the monocyte supernatant. About 5% of the cytostatic activity was recovered after purification. CF constitutes a population of proteins heterogeneous with respect to ionic charge and differing in their isoelectric points, since CF eluted as a broad peak both upon ion-exchange chromatography and chromatofocusing. The isoelectric points of the CF proteins were determined by chromatofocusing to be between 6.0 and 5.0. The molecular weight of CF as determined by gel filtration was in the range of 45,000 to 35,000 daltons. Only one monocyte-secreted protein with a molecular weight of 40,000 was detected upon sodium dodecyl sulphate-polyacrylamide gel electrophoresis of proteins in the purified CF preparations obtained after the final gel filtration step, and this protein may consequently be CF. If the 40,000-dalton protein is CF, one may estimate that about 10(6) monocytes produced roughly 0.1 microgram CF upon activation and that significant cytostasis may be detected with less than nanogram quantities of CF.