Summary:  The molecules that regulate bone cell development, particularly at the early stages of development, are only partially known. Data are accumulating that indicate a complex relationship exists between B cells and bone cell differentiation. Although the exact nature of this relationship is still evolving, it takes at least two forms. First, factors that regulate B-cell growth and development have striking effects on osteoclast and osteoblast lineage cells. Similarly, factors that regulate bone cell development influence B-cell maturation. Second, a series of transcription factors required for B-cell differentiation have been identified, and these factors function in a developmentally ordered circuit. These transcription factors have unpredicted, pronounced, and non-overlapping effects on osteoblast and/or osteoclast development. These data indicate that at least a regulatory relationship exists between B lymphopoiesis, osteoclastogenesis, and osteoblastogenesis.