Human early and late granulocyte-monocyte progenitors (granulocyte-macrophage colony-forming units, CFU-GM), depleted of accessory cells, were physically separated using an antimyeloid monoclonal antibody (DS1.1). They were separately cultured at optimal growth conditions and tested for responsiveness to prostaglandin E2 (PGE2), recombinant tumor necrosis factor alpha (TNF alpha), and transforming growth factor beta-1 (TGF beta 1). Late (DS1.1+) CFU-GM displayed the highest sensitivity to PGE2 and TNF alpha, the first significant inhibition being evident at 10(-9)M PGE2 and 1 U/ml TNF alpha. Conversely, their growth was stimulated (211%-217%) by 0.25-2.5 ng/ml TGF beta 1. Early (DS1.1-) marrow CFU-GM evidenced a lower sensitivity to PGE2 and TNF alpha. Their growth, however, was inhibited by 0.25-2.5 ng/ml TGF beta 1. Early CFU-GM constitute the totality of peripheral blood myeloid progenitors. Cord blood CFU-GM were also demonstrated here to be entirely DS1.1-. Both adult and cord blood CFU-GM displayed the highest resistance to PGE2 and TNF alpha. By contrast, they showed the maximum sensitivity to growth inhibition by TGF beta 1, active at 0.025-0.25 ng/ml. For the first time, therefore, highly purified subsets of human CFU-GM were separated that displayed a different responsiveness to well-defined growth-regulatory molecules. Our results indicate that TGF beta 1 has a dual activity; it is inhibitory on early and stimulatory on late CFU-GM, whereas PGE2 and TNF alpha preferentially inhibit late CFU-GM growth.