Previous studies have suggested a role for c-fos in cellular differentiation in fetal membranes1–3, haematopoietic cells^4,5 and teratocarcinoma stem cells⁶. In other cell types, such as fibroblasts, c-fos expression is normally very low, but is rapidly induced by peptide growth factors, implicating c-fos in growth control mechanisms^7–10. Here, we show that the TPA (12-O-tetradecanoylphorbol-13-acetate)-induced macrophage-like differentiation of HL60 human promyelocytic precursor cells^11,12 is accompanied by the induction of both c-fos mRNA and protein within 15 min after treatment, suggesting a functional role for c-fos in this differentiation system. In quiescent terminally differentiated macrophages, expression of c-fos is inducible by the macrophage-specific growth factor colony-stimulating factor-1 (CSF-1)¹³. The kinetics of c-fos induction, however, are entirely different from those in growth factor-stimulated fibroblasts, supporting the view that the c-fos gene product may serve different functions in different cell types.