Primary human bone marrow megakaryocytes were studied for their ability to express and release cytokines potentially relevant to their proliferation and/or differentiation. The purity of the bone marrow megakaryocytes was assessed by morphologic and immunocytochemical criteria. Unstimulated marrow megakaryocytes constitutively expressed genes for interleukin-1 beta (IL-1 beta), IL-6, granulocyte-macrophage colony-stimulating factor (GM-CSF), and tumor necrosis factor-alpha (TNF-alpha), by the polymerase chain reaction (PCR) and Northern blot analysis. At the protein level, megakaryocytes secreted significant amounts of IL-1 beta (53.6 +/- 3.6 pg/mL), IL-6 (57.6 +/- 15.6 pg/mL), and GM-CSF (24 +/- 4 pg/mL) but not TNF-alpha. Exposure of human marrow megakaryocytes to IL-1 beta increased the levels of IL-6 (87.3 +/- 2.3 pg/mL) detected in the culture supernatants. Transforming growth factor- beta was also able to stimulate IL-6, IL-1 beta, and GM-CSF secretion, but was less potent than stimulation with phorbol-12-myristate-13- acetate (PMA). The secreted cytokines acted additively to maintain and increase the number of colony-forming unit-megakaryocytes colonies (approximately 35%). These studies demonstrate the production of multiple cytokines by isolated human bone marrow megakaryocytes constitutively or stimulated in vitro. The capacity of human megakaryocytes to synthesize several cytokines known to modulate hematopoietic cells supports the concept that there may be an autocrine mechanism operative in the regulation of megakaryocytopoiesis.