We have investigated c-myc, max and c-fos mRNA and protein expression in proliferating, quiescent and stimulated immortalized, SV40 T antigen (LT) transformed and tumor-derived epithelial cells as well in human primary keratinocytes and have compared them to their expression in fibroblasts. In proliferating immortalized and tumor-derived epithelial cells, the levels of c-myc, max and c-fos expression were comparable and much higher than in transformed fibroblasts. c-myc and c-fos mRNA and protein levels remained high even during quiescence, when cells stopped dividing. In contrast, whereas max mRNA was constitutively expressed, max protein levels decreased in both fibroblasts and epithelial cells at high cell density. Changing the medium to serum-free medium of confluent epithelial cells induced a complete proliferative response which started with a transient increase in c-fos and c-myc mRNA, followed by the expression of max. Addition of serum to the medium did not induce additional effects. In fibroblasts, similar treatment induced the arrest of c-myc expression and growth, but max expression was also induced in these cells by serum. Our results therefore show that max expression is growth regulated in both immortalized and transformed epithelial as well as fibroblast cells. In contrast, in epithelial cells, c-myc displayed two contrasting behaviors.