We studied the therapeutic effect ofLactobacillus caseiduring an infection withSalmonella typhimuriumand the influence of the administration ofL. caseion the levels of secretory immunoglobulin A (S‐IgA) specific for the pathogen. Swiss albino mice were infected with S.typhimurium,different doses ofL. caseibeing orally administered to the animals 48 h after infection. Results showed that the dose ofL. caseiused for therapeutic purposes was extremely important, since the only partially effective one was that used for 3 consecutive days (3.6 ×109CFUs) producing an S‐IgA level slightly higher than that of the controls. Larger doses and longer feeding periods (4, 5 and 7 consecutive days) failed to improve the therapeutic effect and proved unable to check the infection. When a repeated stimulation withL. caseiwas effected in those animals that had undergone the 3‐day treatment, an enhancement in the S‐IgA levels was obtained and their survival time increased to 35 days as against 20 days for the controls without treatment. The stimulation of S‐IgA synthesis lasted only a short time after the administration of the optimum protective dose ofL. casei.Although the IgA levels obtained on the 3rd and 5th days after feeding were high in relation to those of the controls, they were not sufficient to prevent colonization. It seems likely thatL. caseistimulated the T‐ and/or B‐cells, enhancing the number of B cells that enter the IgA cycle and thus increasing the synthesis of immunoglobulin A. This stimulation is apparently closely related to theL. caseidose.