An isopropanolic extract (iCR) from the rhizome of black cohosh is used in the treatment of menopausal symptoms. Whereas animal studies in rats also suggest positive skeletal effects for iCR, the mechanism of its actions on bone cells remains unclear. RANKL is essential for osteoclast formation and activation, while osteoprotegerin (OPG) neutralises RANKL. Therefore, we assessed the effects of iCR on OPG and RANKL mRNA steady state levels by semiquantitative RT-PCR and on protein production by an ELISA system in primary human osteoblasts (hOBs). Treatment with iCR increased OPG mRNA levels and protein secretion of hOBs by 2- to 3-fold in a dose-dependent manner, with a maximum effect at a 106-fold dilution of iCR (p < 0.001) after 24-48 h. Time-course experiments indicated a stimulatory effect of iCR on osteoblastic OPG protein secretion by 3- to 5-fold (p < 0.001) as early as 12 h, whereas RANKL expression was very low and was not found to be modulated by iCR. Moreover, iCR enhanced two osteoblastic differentiation markers, bone-specific alkaline phosphatase activity and osteocalcin expression, by up to 4- and 3-fold, respectively (p < 0.001). Interestingly, the pure estrogen receptor antagonist ICI 182,780 abrogated the stimulatory effect of iCR on OPG production. This proposes estrogen receptor-mediated pathways for black cohosh and iCR could thus qualify as a tissue specific estrogen agonist and/or antagonist. Our data suggest that iCR enhances differentiation and increases the OPG-to-RANKL ratio of normal human osteoblasts. These effects may contribute to the positive skeletal effects of black cohosh.