Ipriflavone, a synthetic isoflavone derivative, reduces bone resorption by inhibiting osteoclasts activity. In order to evaluate the role of Ipriflavone on osteoclast growth and differentiation, we tested Ipriflavone and its four "in vivo" main metabolites (Metabolites I, II, III, and V) on a clonal population of human osteoclast precursor cells (FLG 29.1). Pharmacological doses of Ipriflavone and Metabolite III were able to inhibit cell proliferation and interleukin 6 release. In co-cultures of FLG 29.1 cells and osteoblastic (Saos-2) cells Ipriflavone at 1 microM dose inhibited the adhesion of FLG 29.1 cells to the osteoblastic monolayer and reduced the immunocytochemical reaction of the vitronectin receptor. Binding studies with tritiated Ipriflavone showed the presence of a single specific binding site, wtih a Kd of about 70 nM and a binding capacity of 8 fmol/10(6) cells. These results demonstrate a direct effect of Ipriflavone and of Metabolite III on the human osteoclast precursor cell line FLG 29.1.