The ability of cultured human fibroblasts to synthesize and secrete a mitogenic, somatomedin C-like peptide suggests that human fibroblasts unlike certain other cell lines may not require exogenous somatomedin for cell proliferation. The aim of this study was to examine the effects of hypophysectomy and subsequent growth hormone (GH) replacement on the ability of rat serum to stimulate proliferation and DNA synthesis in cultured human fetal skin fibroblasts. For comparison we have also examined the effects of serum from hypophysectomized (hypox) and GH-replaced hypox rats on mouse BALB/c 3T3 fibroblasts, a cell line which has been shown to require exogenous somatomedin (SM) for DNA synthesis and cell proliferation. Human fibroblast proliferation and DNA synthesis (measured as thymidine incorporation) were significantly enhanced (P less than 0.025) when these cells were cultured in low concentrations of hypox rat serum compared to normal rat serum. When thymidine incorporation into DNA was expressed in terms of a normal rat serum pool (arbitrarily assigned a potency of 1 unit/ml), serum from hypox rats was significantly more potent than serum from normal rats (1.52 +/- 0.16 (SEM) vs. 0.99 +/- 0.10 units/ml, P less than 0.0125). Replacement treatment of hypox rats with human GH, 0.2 IU/100 g body weight/day or 0.4 IU/100 g body weight/day decreased the potency of the rat serum in terms of stimulation of thymidine incorporation from 1.52 +/- 0.16 to 1.30 +/- 0.14 and 1.08 +/- 0.06 units/ml respectively. In contrast serum from untreated hypox rats was significantly less potent than normal rat serum in stimulating BALB/c 3T3 DNA synthesis (0.36 +/- 0.06 vs. 1.04 +/- 0.16 units/ml, P less than 0.0025).(ABSTRACT TRUNCATED AT 250 WORDS)