Raloxifene is a selective estrogen receptor modulator (SERM) that prevents bone loss. Although it is largely used for the treatment of osteoporosis, the mechanisms by which this compound modulates the activity of bone cells are still poorly understood. In this study we investigate whether raloxifene affects osteoclast and osteoblast activity in vitro. Bone marrow cultures were established from neonatal mice and treated with 1,25(OH)(2) vitamin D(3) (VitD(3), 10(-8) mol/L) to induce osteoclast generation. Similar to 17beta-estradiol, raloxifene significantly reduced the number of osteoclasts in a concentration-dependent manner, with maximal inhibition at 10(-11) mol/L (-48%). However, as for 17beta-estradiol, at a high concentration (10(-7) mol/L), the inhibitory effect of raloxifene was abolished. In a pit assay, raloxifene inhibited bone resorption. A maximal effect was observed at 10(-9) mol/L, and maintained at a high concentration, indicating that inhibition of osteoclast formation and inhibition of bone resorption may be due to activation of, at least in part, different pathways. Osteoblasts from neonatal mice calvariae were also exposed to raloxifene. In these cells, this compound induced a concentration-dependent increase of proliferation, which was blocked by the estrogen-receptor antagonist ICI 164,384. Raloxifene also increased the osteoblast-specific transcription factor Cbfa1/Runx2 and alpha2 procollagen type I chain mRNAs, with a pattern that only partially coincided with that of 17beta-estradiol. Consistent with decreased osteoclastogenesis, raloxifene inhibited the mRNA expression of interleukin (IL)-1beta and IL-6 at a low concentration, but not at a high concentration, whereas 17beta-estradiol had similar effects on IL-6 and inhibited IL-1beta at both concentrations. Furthermore, both compounds were able to inhibit tumor necrosis factor (TNF)-alpha-induced IL-1beta, but not IL-6, increase. In conclusion, these data show that raloxifene negatively modulates osteoclasts, and positively affects osteoblasts, suggesting not only an antiresorptive role, but also an osteoblast stimulatory role.