The influence of transforming growth factor-beta (TGF-beta) on hematopoiesis has been evaluated by adding blocking antibodies against TGF-beta to colony forming assays (CFU-c). When optimum concentrations of recombinant growth factors, granulocyte-macrophage colony stimulating factor (GM-CSF), and interleukin-3 (IL-3) were added to stem cells from the peripheral blood of healthy individuals and certain patients with tumors or HIV infection, the anti-TGF-beta capable of blocking 5 ng/ml of active TGF-beta had no significant influence on erythroid or myeloid colony formation. However, in certain immunosuppressed individuals, anti-TGF-beta resulted in a significant decrease of erythroid colony formation and slight suppression of myeloid colony formation. The significant inhibition of hematopoiesis by plasma of HIV patients could be due to the presence of active forms of TGF-beta. The results of the blocking experiments are consistent with the concept that TGF-beta in low concentrations is essential for erythropoiesis and myelopoiesis but that higher levels of TGF-beta primarily inhibit erythropoiesis in vitro. TGF-beta serves as a coordinating factor when efficient recruitment of granulocytes and monocytes is more essential than erythropoiesis and stem cell growth.