Recent in vitro findings suggest that bisphosphonates, potent inhibitors of osteoclastic bone resorption, may also have a direct action on osteoblasts. The purpose of this study was to search for potential effects of etidronate and alendronate on the formation of early and late osteoblastic cell precursors by measuring the number of colony-forming units for fibroblasts (CFU-F) and colony-forming units for osteoblasts (CFU-OB) in murine and human bone marrow cultures. In murine marrow cultures, etidronate (10⁻⁵ to 10⁻⁹ mol/L) significantly stimulated the formation of CFU-F with a maximal effect at 10⁻⁵ mol/L (mean increase over control values ± SD: 106 ± 17%; p < 0.001), whereas alendronate had a biphasic effect, being stimulatory at concentrations below 10⁻⁷ mol/L (78 ± 5%; p < 0.001), and inhibitory at higher doses. The formation of CFU-OB was also inhibited by both bisphosphonates at the highest concentrations (10⁻⁵ mol/L and 10⁻⁶ mol/L), but it was significantly stimulated at lower concentrations (from 10⁻⁷ to 10⁻⁹ mol/L for etidronate and 10⁻⁷ to 10⁻¹⁰ mol/L for alendronate; p < 0.001). In human bone marrow cultures, alendronate (10⁻⁸ to 10⁻¹² mol/L) increased CFU-F formation with a maximal effect at 10⁻¹⁰ mol/L (161 ± 12%; p < 0.01). CFU-OB formation, observed only in the presence of dexamethasone (10⁻⁸ mol/L), was markedly stimulated by alendronate at the above concentrations with a maximal increase at 10⁻¹⁰ mol/L (133 ± 34%; p < 0.001). The in vivo short-term effects of bisphosphonates on the formation of early osteoblast precursors were also studied in bone marrow cultures from young female mice treated with weekly subcutaneous injections of etidronate (0.3, 3, and 30 mg/kg) or alendronate (0.3, 3, and 30 μg/kg) and from aging female mice treated with the two lowest doses of both drugs. After 1 month of treatment, etidronate (0.3 and 3 mg/kg) and alendronate (0.3 and 3 μg/kg) significantly increased the number of CFU-F colonies in the bone marrow from young and old animals, whereas the highest dose of both drugs had no effect in young mice. Our results, together with previously reported observations of bone-forming effects in osteoporosis, suggest that bisphosphonates may have, in vivo, a potentially relevant influence on cells of the osteoblastic lineage, distinct from their inhibitory action on osteoclasts.