Abstract: The stromal cells that support blood cell production within bone marrow are pre-adipocytes and functional interactions with marrow fat cells have long been suspected. Adiponectin was recently isolated as an adipocyte product and shown to have structural similarities to Clq as well as members of the TNF superfamily. It suppresses myeloid differentiation in short term bone marrow cultures and also inhibits macrophage functions. These observations raised the possibility that precursors of other blood cell lineages interact with fat cells in marrow via adiponectin. It has now been determined that the factor blocks B lymphopoiesis in Whitlock-Witte type bone marrow cultures, but not the production of myeloid cells in Dexter cultures. Several observations suggest that non-lymphoid cells represent the target of this new mediator, and the B lymphoid lineage is only indirectly influenced. Highly purified lymphocyte precursors in stromal cell-free, serum-free cultures were unaffected by adiponectin.