B lymphopoiesis declines with age, and in rabbits this occurs by 8 wk of age. We found that CFU fibroblasts (CFU-Fs) in the bone marrow (BM) decrease 10-fold by a few weeks of age and that the CFU-Fs preferentially differentiate into adipocytes instead of osteoblasts. BM becomes filled with fat spaces during this time, making rabbit a unique model to study the effects of accelerated fat accumulation on B lymphopoiesis. We show that adipocytes of both rabbit and human secrete a soluble factor(s) that inhibits B lymphopoiesis, and we tested if this inhibition was due to effects on the BM stroma or hematopoietic progenitors. Pretreatment of BM mononuclear cells with adipocyte conditioned medium dramatically inhibited their differentiation into proB cells in cocultures with OP9 stromal cells. In contrast, pretreatment of OP9 stromal cells with adipocyte conditioned medium had no effect on B lymphopoiesis. Using human hematopoietic stem cells, we show that inhibition by the adipocyte-derived factor occurred at the common lymphoid progenitor to preproB cell stage. We propose that the age-related decline in B lymphopoiesis is due to a decrease in CFU-Fs, an increase in adipocytes, and an adipocyte-derived factor that blocks B lymphopoiesis at the common lymphoid progenitor to preproB cell stage.