Namalva, a human B cell lymphoma line, produced a factor with a molecular weight of approximately 60,000 which enhanced the proliferation of normal activated human B lymphocytes. The factor also enhanced the proliferation of certain B cell lines. It can be distinguished physiologically and biochemically from other lymphokines known to enhance B cell proliferation, namely, interleukin (IL) 1, IL 2, and interferon. The production of B cell growth factor by B cell tumor lines may contribute to their ability to grow autonomously and may reflect an important component of the neoplastic potential of the cell. B cell growth factor produced by tumors may also affect normal cells in vivo.