This study was based on an initial observation that, although culture of T cells from Salmonella-infected mice with concanavalin A induced both L3T4+ T cells and Lyt-2+ T cells to proliferate, there was a relative increase in the responsiveness of the Lyt-2+ T cells in suspensions harvested from mice with secondary infection. Accordingly, primed T cells, obtained from the peritoneal cavities and spleens of mice that had received one or two intraperitoneal doses of Salmonella were examined for the presence of antigen-specific, class I major histocompatibility complex (MHC)-restricted Lyt-2+ T cells. After primary infection with avirulent Salmonella enteritidis 11RX (11RX) only L3T4+ T cells could be induced to proliferate in response to formalin-killed 11RX organisms, and a second dose of live 11RX did not change the phenotype of the responding T-cell population. In contrast, secondary challenge with S. typhimurium C5 (C5) generated cell populations where both L3T4+ and Lyt-2+ T cells proliferated when cultured with formalin-killed 11RX. Transfer of delayed-type hypersensitivity (DTH) using mixtures of primed T cells and either killed or live Salmonella organisms demonstrated that DTH was mediated by L3T4+ T cells, and secondary infection with either the 11RX or C5 strain did not change this result. However, antigen-specific Lyt-2+ T cells which mediated DTH reactivity were detected using a Salmonella-infected cell line which expressed MHC-coded class I but not class II products. These Lyt-2+ T cells were present in the spleen and peritoneal cavity after secondary infection and in the peritoneal cavity late after a primary infection with 11RX.