Protective effects of ipriflavone, an isoflavone derivative, in osteoporosis are believed to be caused by the inhibitory action on bone resorption. A direct effect of ipriflavone on bone formation is as yet unknown. Ipriflavone and four of its metabolites (I, II, III, and V) were examined for their effects on parathyroid hormone response, collagen synthesis, alkaline phosphatase activity, and cell proliferation in a clonal cell population of rat osteoblastic cells. Pretreatment of osteoblasts with high concentrations of ipriflavone for 48 h significantly inhibited the cAMP response to parathyroid hormone, producing a shift in the dose-response curve; at lower concentrations metabolites II and III potentiated the cAMP accumulation induced by low doses of parathyroid hormone. The 48 h treatment with metabolite V at the 1 nM dose significantly stimulated collagen synthesis in osteoblastic cells. Ipriflavone and metabolite I showed a biphasic stimulatory action on the alkaline phosphatase activity of osteoblasts, with a maximal effect at the 0.1 and 1 nM doses, respectively. A similar biphasic response was observed with ipriflavone and metabolite I on osteoblastic cell growth, with a maximal effect at the 0.1 nM concentration. These results suggest a direct role of ipriflavone in modulating the synthetic and growth properties of osteoblast-like cells.