We have recently described a human T cell line, named PF-382, obtained from the pleural effusion of a child with T-acute lymphoblastic leukemia (T-ALL), which expresses phenotypic and functional features of suppression. In this study we report that PF-382 spontaneously releases a factor which inhibits the in vitro growth of myeloid (CFU-GM) and erythroid (BFU-E) progenitor cells. The same effect is obtained when irradiated PF-382 cells are co-cultured with the hemopoietic precursors. In both instances, maximal inhibitory activity is exerted on day 14 CFU-GM and BFU-E obtained from the light density nonadherent fraction of normal human bone marrow and peripheral blood; this finding suggests that the target of the inhibition is represented by the more immature elements within the progenitor cell compartment. Progressive depletion of monocytes, T, B lymphocytes, and NK cells as well as recloning experiments indicate that the inhibitory effect is directly exerted on the target cell and not via an intermediate population of accessory cells. Partial purification by gel filtration and by subsequent high performance liquid chromatography demonstrates that this factor is a protein with a molecular weight of 47 kd. The physicochemical characterization and the specific functional properties suggest that the PF-382 inhibitory factor represents a lymphokine which differs from those so far reported. The PF-382 cell line provides a useful model toward a better understanding of the interrelations between T cell subsets and other hemopoietic compartments.