Epidermal growth factor (EGF) stimulates both DNA and RNA synthesis in contact-inhibited human fibroblasts. Stimulation of DNA synthesis is observed at concentrations as low as 3 pM, is half-maximal at 70 pM, and is maximal at 300 pM EGF. The action of EGF is similar to that of fetal-calf serum, but is distinguished by the time-course of stimulation and by the ability of serum to stimulate further those cells maximally stimulated by EGF. Cells that synthesize DNA in response to physiological concentrations of EGF (10(-11) to 10(-10) M) are insensitive to physiological concentrations of insulin (10(-11) to 10(-10) M) and respond only minimally to very high concentrations of this hormone (10(-6) M). The biological activity of EGF is paralleled by binding of this peptide to fibroblasts in a specific and saturable manner; the dissociation constant is about 800 pM. The binding of EGF is unaffected by either insulin or cholera toxin. Cholera toxin inhibits the action of both EGF and serum. Suppression of DNA synthesis is observed at 0.02 pM toxin, and is maximal at about 2 pM. Cells treated with cholera toxin at these concentrations appear to be otherwise viable by several criteria. The stimulatory effects of EGF are also inhibited by theophylline and dibutyryl cyclic AMP separately or in combination. These observations indicate that fibroblasts possess receptors for EGF by biological and physicochemical criteria, and suggest that a similar if not identical peptide may be amongst those factors in sera which stimulate cell growth. The possibility is considered that EGF and cholera toxin modulate the ability of a cell to initiate polynucleotide synthesis by way of specific cell-surface interactions which in turn alter the levels of intracellular cyclic AMP.