Previous studies have shown that phorbol ester induced macrophage differentiation in the leukemic cell line U-937 is tightly linked to the expression of the c-jun protooncogene and the generation of AP-1 transcriptional activity. We expressed the c-jun protooncogene in U-937 cells to examine the role of c-jun in the differentiation of these cells. AP-1 DNA binding activity and the expression of AP-1 controlled downstream genes were increased in all clones expressing exogenous cJun. More importantly, these transfectants showed evidence of differentiation as measured by the acquisition of phagocytic capacity, although they did not develop morphological changes associated with differentiation such as adherence. Furthermore, they became committed to terminal differentiation after significantly shorter exposures to phorbol esters than did control cell lines. These data show that cJun expression induces partial differentiation along the macrophage pathway in U-937 cells.