Although there may be a close relationship between B lymphocytes and osteoclasts, or bone resorbing cells, little is known about the role of B lymphocytes in bone formation. We compared in vivo new bone induction in mice homozygous for the B-cell deficient (mMT) gene knockout, which lack functional B lymphocytes, with bone induction in control wild-type (C57BL/6) mice. Our comparison used two models of new bone induction in vivo: endochondral osteoinduction by subcutaneous implantation of recombinant human bone morphogenetic protein (rhBMP-2) and osteogenic regeneration after tibial bone marrow ablation. The expression of bone-specific proteins (bone sialoprotein, osteopontin, and osteocalcin) and inflammatory/immunomodulatory cytokines (interleukin-1a and -1b, interleukin-6, and tumor necrosis factor-a) was assessed by Northern blot analysis or reverse transcription-polymerase chain reaction, respectively. Ossicles induced by rhBMP-2 were larger in volume and mass in mMT knockout mice, but relative volumes of the newly induced bone, cartilage, and bone marrow were similar in the two groups. Six days after tibial bone marrow ablation, mMT knockout mice resorbed the initial blood clot faster and formed more trabecular bone, paralleled by greater levels of bone sialoprotein mRNA than in the wild-type mice. mMT knockout and wild-type mice also differed in the expression pattern of inflammatory/immunomodulatory cytokines during the development of the newly induced bone, suggesting that a genetic lack of B lymphocytes may create a change in the immunological milieu at the site of new bone induction, which stimulates the initial accumulation and proliferation of mesenchymal progenitor.