The cellular response in the course of experimental infection with Salmonella typhimurium was studied in mice. T cells were detected by the presence of theta-antigen, B cells by the binding of fluorescent immunoglobulins, and cells with receptors by labeled Salmonella binding. Lymphocytes were from spleen and lymph nodes. Results have been divided into three groups: group A, including mice with slight symptomatology; group B, including those with serious infection symptomatology; and group C, including mice that died in the course of the experiment. In spleen and lymph nodes of group A mice, an increase in the percentage of T and B lymphocytes was observed. This increase reached a peak 10 days after experimental infection. In lymph nodes, the B-cell percentage was equal to the percentage of T cells, whereas in spleen lymphocytes the B-cell percentage was higher. In spleens of group B mice we observed the same response as in mice of group A, whereas in lymph nodes there was a low response of T and B lymphocytes. In group C mice, there was no significant response of T and B lymphocytes in either spleen or lymph nodes. In B lymphocytes prepared from spleens of surviving mice, a small number of Salmonella receptors was detected: 200 bacterial cells per 10(9) lymphocytes.