Purified populations of bone marrow-derived (B-) lymphocytes and thymus-derived (T-) lymphocytes were obtained from C3D2F1 hybrid mice shown to be immune to Salmonella typhimurium. These subpopulations of lymphocytes were injected into normal mice; four days later the animals were challenged with 50 50% lethal doses of S. typhimurium, and viable bacteria in livers, spleens, and blood were counted at various intervals after challenge. On day 8 after challenge, the mice supplemented with B-lymphocytes showed a significant decrease in the number of organisms recovered from all three sites, compared with that seen in recipients of T-lymphocytes and in controls. The mice given B-lymphocytes showed a better rate of survival (65%) than mice that received only T-lymphocytes (21%) or T-lymphocyte fractions contaminated 10%-30% with B-lymphocytes (49%). These data indicate that, although the humoral response is not totally protective, it does play an important role in the suppression of the infection during its early stages.