A growth factor was found in the supernatants of MOLT-4f, a cell line derived from acute T lymphoblastic leukemia. This factor, which we designated leukemia-derived growth factor from MOLT-4f (LDGF-M4), is different from interleukin 2. LDGF-M4 has features of a polypeptide with a molecular weight in the range of 5,000-15,000, as indicated by gel diffusion chromatography. LDGF-M4 does stimulate MOLT-4f and at least two other T cell lines that do not respond to interleukin 2. Because MOLT-4f cells produce and respond to LDGF-M4, this factor may contribute to the independence of MOLT-4f and related T leukemia cell lines.