Natural cell-mediated cytotoxicity (NCMC) like immune T cell-mediated cytotoxicity and antibody production is regulated by a soluble factor released during coculture of lymphocytes with mitomycin C-treated lymphoblastoid cell lines. This N-cell-activating factor (NAF) enhances the activity of effector N cells and increases natural cytotoxicity. There appears to be no restriction for compatibility at the A and B locus of the major human histocompatibility complex in the production or activity of the factor. NAF was observed in the supernatant as soon as 2 days after initiation of mixed culture with a peak of production at 5 days. A soluble factor produced and released by T cells in response to stimulation by other cells acts by enhancing cytotoxicity of effector cells in NCMC, demonstrating a T-N cell cooperation.