PRIP (phospholipase C-related, but catalytically inactive protein) is a novel protein isolated in this laboratory. PRIP-deficient mice showed increased serum gonadotropins, but decreased gonadal steroid hormones. This imbalance was similar to that for the cause of bone disease, such as osteoporosis. In the present study, therefore, we analyzed mutant mice with special reference to the bone property. We first performed three-dimensional analysis of the femur of female mice. The bone mineral density and trabecular bone volume were higher in mutant mice. We further performed histomorphometrical assay of bone formation parameters: bone formation rate, mineral apposition rate, osteoid thickness, and osteoblast number were up-regulated in the mutant, indicating that increased bone mass is caused by the enhancement of bone formation ability. We then cultured primary cells isolated from calvaria prepared from both genotypes. In mutant mice, osteoblast differentiation, as assessed by alkaline phosphatase activity and the expression of osteoblast differentiation marker genes, was enhanced. Moreover, we analyzed the phosphorylation of Smad1/5/8 in response to bone morphogenetic protein, with longer phosphorylation in the mutant. These results indicate that PRIP is implicated in the negative regulation of bone formation.