Human natural interferon gamma (IFN-gamma) exerts differentiating and cytocidal effects in vitro on the human myeloblastic leukemia cell line ML-2. The anti-leukemic effects of the crude preparation of IFN-gamma are more intensive than those of the mixture of purified IFN-gamma, IFN-alpha, tumor necrosis factor alpha and lipopolysaccharide, suggesting existence of an unknown anti-leukemic factor in the crude IFN-gamma sample.