Virus-specific cytotoxic T lymphocytes (CTLs), which kill virus-infected cells, are thought to be a major host defense against viral infections. The addition of interleukin 7 (IL-7) at the onset of mitogen-stimulated cultures resulted in a marked (up to threefold) augmentation of env-specific cytotoxicity in human immunodeficiency virus type 1 (HIV-1)-infected individuals (p < 0.001). Addition of IL-7 on day 3 or 5 produced a significant but lesser augmentation of CTL response as compared to day 0. The IL-7-induced proliferative response and augmentation of cytotoxic activity was time and dose dependent, with an optimal IL-7 concentration of 1000 U/ml. Cell surface phenotypic analysis of CTL effector cells indicates that IL-7 primarily affects the proliferation of CD8⁺ T cells. Anti-IL-2 monoclonal antibody (MAb) substantially inhibited the proliferative effect of IL-2, but did not affect the proliferative effect of IL-7. Endogenous IL-2-induced generation of cytotoxic T cells was blocked by MAbs to IL-2 or IL-2R. The addition of IL-7 restored the process of conversion of precursor CTLs (pCTLs) to mature CTLs (mCTLs) and significantly enhanced specific cytolytic activity. It appears that IL-7 is a potent regulatory cytokine capable of acting independently of IL-2 in mitogen-specific activation of pCTLs to mCTLs. These data suggest that IL-7 should be considered as a potential therapeutic approach in AIDS and other infectious diseases in which CTL response declines.