Human myeloid colony formation in diffusion chambers in mice (CFU-DG) was enhanced following administration of a human T-cell-line-derived conditioned medium (Mo). The Mo cell line also elaborates activities stimulating human myeloid colony formation in vitro in agar (CSF) and potentiating erythroid colony formation in vitro in methylcellulose (EPA). Depletion of CSF from Mo conditioned medium by heat inactivation or gel exclusion chromatography did not affect CFU-DG formation. EPA and CFU-DG stimulating activities are heat stable and have approximately the same molecular weight. Culture of human bone marrow cells in diffusion chambers in mice for 4 days under the influence of Mo conditioned medium resulted in significant increment of BFU-E and CFU-DG as judged by subculture of diffusion chamber contents. No effect on CFU-C could be detected.