Both insulin-like growth factor-I (IGF-I) and transforming growth factor beta (TGF beta) have powerful modulatory effects in a variety of tissues. A major target of action is the skeletal system, where they enhance bone formation and decrease matrix degradation, thus playing a part in the maintenance of bone mass. Because of the potent mitogenic effect of these agents on osteoblasts, recombinant IGF-I (rhIGF-I) and recombinant TGF beta (rhTGF beta) have potential as drugs to stimulate bone formation in the prevention and treatment of osteoporosis. Using biochemical markers, subcutaneous rhIGF-I therapy has been shown to increase bone turnover and bone formation in nonosteoporotic older people. However, a corresponding increase in bone mass has not yet been documented nor have there been reports yet on the effects of systemically administered rhTGF beta in humans. Further investigation is required to define the clinical potential of rhIGF-I and rhTGF beta as therapeutic agents in age-related osteoporosis.