Stimulation of the growth of quiescent fibroblasts by polypeptide growth factors is accompanied by the rapid induction of the c-fos proto-oncogene. To investigate whether there exists a relationship between mitogenic activity and c-fos expression, we analysed cellular responses (DNA synthesis and cell growth) and c-fos gene induction (mRNA and proteins) in a rat embryo fibroblast line (EL2) stimulated with epidermal growth factor (EGF), fibroblast growth factor (FGF), 12-O-tetradodecanoyl phorbol-13-acetate (TPA) and transforming growth factor beta (TGF beta). Our results suggest that the susceptibility of EL2 cells to a growth factor could be predicted as a function of the c-fos expression caused by the same growth factor. These also indicate that the c-fos gene expression may have contributed to moving our cells out of the quiescent state, but it is not the only essential event required to effect EL2 cell growth.