Innately resistant (Ityr) A/J mice infected with the virulent Salmonella typhimurium C5 strain suppress the early exponential bacterial growth in the reticuloendothelial system toward the end of the first week of infection, with spleen and liver bacterial counts reaching a plateau phase. In vivo administration of neutralizing anti-interleukin-12 (IL-12) antibodies did not affect early bacterial growth in the tissues (days 1 to 3) but impaired the establishment of the plateau, with higher spleen and liver counts by day 7 of the infection in anti-IL-12 treated mice than in untreated controls. Gamma interferon (IFN-gamma) was detectable in the sera and spleen homogenates of both control and anti-IL-12-treated mice on days 3 and 7 of the infection. Noticeably, IFN-gamma levels were significantly lower in anti-IL-12 treated mice than in control animals. Splenocytes from uninfected A/J mice released IFN-gamma in response to concanavalin A (ConA) or to S. typhimurium C5. In vitro IL-12 neutralization dramatically impaired the IFN-gamma response to S. typhimurium but not to ConA. Splenocytes harvested from infected anti-IL-12 treated mice on day 7 of the infection produced significantly lower amounts of IFN-gamma upon in vitro stimulation with ConA and with a Salmonella protein-rich extract than did cells from similarly infected untreated control animals. Spleen cells from infected mice showed lower proliferative (mitogenic) responses to ConA and to a Salmonella soluble extract than did cells from uninfected mice. In vivo anti-IL-12 treatment significantly restored the ability of splenocytes from infected mice to proliferate in response to the antigens and ConA. In vivo neutralization of IL-12n in innately susceptible BALB/c mice ((ItyS)) immunized with a live attenuated aromatic-dependent Salmonella vaccine reduced host resistance to virulent oral challenge with S. typhimurium C5. Thus, in primary Salmonella infections, IL-12 mediates the suppression of growth of virulent salmonellae in the reticuloendothelial system, positively modulates IFN-gamma production, and is involved in the immunosuppression which accompanies the acute stages of the disease. IL-12 also contributes to host resistance to virulent organisms in secondary infections.