Gamma-interferon (gamma IFN), like the phorbol ester TPA, is able to induce the differentiation of the HL 60 human promyelocytic cell line in the monocytic pathway. Morphological and serological data show a differential expression of cell surface markers upon treatment with these two inducers. It was reported that TPA treatment alters the expression of some protooncogenes, like c-myc and c-fos, involved in the induction of cell proliferation, and c-fos and c-fms (CSF.1 receptor), linked to monocytic differentiation. We have analyzed the variations in the levels of expression of these oncogenes upon gamma IFN treatment of HL 60 cells. c-myc expression was unchanged during the whole induction period. The early increase reported in c-fos expression was not observed; however, c-fos levels increased upon 24 hr of gamma IFN treatment. These results suggest either that TPA and gamma IFN act at different steps, or, alternatively, that different pathways exist in the monocytic differentiation.