We tested conditioned media from 12 patients with T lymphocyte neoplasms and four T cell lines for their ability to stimulate the in vitro growth of erythroid-burst-forming units (BFU-E) from bone marrow mononuclear cells in a methylcellulose culture system. Nine patients suffered from acute lymphocytic leukemia, two from chronic lymphocytic leukemia, and one from non-Hodgkin's lymphoma. The T lymphocytes were characterized by a series of monoclonal antibodies and their stage of development was correlated with their ability to produce burst-promoting activity (BPA). Conditioned media from cells classified as prothymocytes (three cases), common thymocytes (one case), mature thymocytes (three cases), and mature lymphocytes of the helper subtype (two cases) increased BFU-E proliferation four- to 19-fold over control values using normal bone marrow as target cells. Conditioned media from OKT8+ malignant T lymphocytes (three cases) did not enhance BFU-E proliferation. Conditioned media from cells classified as immature T cells stimulated CFU-GM proliferation in only one of seven cases even though they secreted BPA. Conditioned media from three of the four cell lines stimulated by phytohemagglutinin, enhanced BFU-E growth. Our results indicate that malignant cells that have characteristics of immature T cells are able to produce BPA. Studies using techniques to isolate homogeneous populations of normal T cell subsets are required to determine whether normal immature T lymphocytes have the same capability.