An in vitro assay developed as a correlate of vaccine-induced protection from human immunodeficiency virus (HIV) was validated in populations with relative resistance to HIV-1 as well as in HIV vaccine recipients. Cultures of peripheral blood mononuclear cells (PBMC) were challenged with 10 TCID50 of HIV-1MN or HIV-1BaL, titered in PBMC from normal controls (n=57). PBMC from HIV-1-infected persons with low viremia (n=17), exposed uninfected persons (n=23), and HIV-2-infected Senegalese prostitutes (n=9) were significantly resistant to HIV-1BaL and/or HIV-1MN (P<.001). Among 34 HIV vaccine recipients of live canarypox vector expressing multiple HIV-1 gene products with or without rgp120 booster, PBMC from postvaccination samples were significantly resistant to both strains (P<.001), and cytotoxic T lymphocyte precursor-positive samples were significantly more resistant than were precursor-negative samples (P<.03). This is the first evidence of the induction by vaccination of a validated correlate of protection. This assay should serve as a useful criterion for assessing experimental HIV vaccines before phase III efficacy trials.