By infecting human leukemia cell lines in vitro with HIV-1IIIB' a number of HIV-1 carrier clones were generated. Among them, 5 of 13 CD8+ HIV-1 carrier T cell clones were shown to acquire a rapid cytotoxic activity (within 1 hour) specific to TdT+CD4+CD8+ immature T cells including normal thymocytes. This novel cytotoxic reaction, without requiring virus infection and indicating a rapid T cell precursor elimination during active lymphopoiesis, suggests a mechanism responsible for mature CD4+ T cell depletion in HIV-1 infected individuals.