Gene-recombinant human growth hormone (rhGH) elicited a dose-dependent stimulation of the proliferation of osteoblast-like cells (OB), when grown in strictly serum-free longterm cultures. A half-maximal effect was observed at concentrations of 15-20 ng/ml and the maximal stimulation was 160% of hormone-free controls. The rhGH-induced effect on proliferation could be inhibited dose-dependently by the addition of an insulin-like growth factor (IGF) I-antiserum to the medium. Moreover, IGF I and rhGH had additive effects only when the exogenous IGF I concentration exceeded that of endogenously produced IGF I by a large margin. Thus, direct stimulation of OB proliferation by rhGH is, at least in part, mediated by IGF I-like immunoreactivity.