Recent studies have shown that soluble factors elaborated by human T lymphocytes enhance erythroid burst formation by human peripheral blood null cells. This study demonstrates that media conditioned by a long- term T lymphocyte line augmented the growth of erythroid colonies in vitro in the presence of erythropoietin (Ep). ATCC.CCl 119 (CCRF-CEM) was derived from a patient with ALL of T-lymphoblast origin. Cells from the stocks used in these experiments maintained T-cell characteristics as determined by histochemical and rosetting techniques. Increased numbers of 16 day BFU-E were seen when Ficoll-Hypaque separated peripheral blood leukocytes were cultured in the presence of a 10% (v/v) concentration of CCL 119 conditioned medium (CM). CM increased the number of BFU-E even when Ep or fetal calf serum were not growth limiting. CM also increased the number of late BFU-E observed in cultures of nonadherent bone marrow cells. When peripheral blood mononuclear cells were depleted of E-rosetting cells, only small numbers of BFU-E grew. Addition of 119 CM increased the numbers of BFU- E in E-rosette-depleted cultures. CM from B-cell, macrophage, or other T-cell lines tested did not stimulate BFU-E growth as consistently. These studies indicate that CM obtained from ATCC.CCL 119 cells contained burst-promoting activity, one of the factors required for proliferation of early erythroid progenitors.