The mechanisms of protective action of Lactobacillus have been studied during development of the generalized infection induced by Klebsiella pneumoniae in CBA mice after weaning. The mice were infected intragastrically during the first day after weaning (1 x 10(9) bacterias per mice). Suspensions of Lactobacillus were introduced before and after infection during 10 days (1 x 10(6) bacterias per mice). It has been shown that introduction of Lactobacillus substantially decreased the level of the gut contamination by Klebsiella, prevented generalization of infection and death of animals. Significant higher levels of IgA in the blood serum, IgA and IgM in the gut content, percentage of splenocytes, expressing surface IgM and IgG were observed on the 7th day as compared with those in animals without Lactobacillus. Significantly lower percentage of splenocytes, expressing CD4 antigen was also observed. On the 11th day after infection the mice receiving lactobacillus have shown a tendency to an increase of IgA in the gut content, significantly lower concentrations of IgM in the gut content and a higher level of IgA to the blood serum as compared with the control. Other characteristics were comparable to those of the control group. A conclusion is made that introduction of Lactobacillus prevents development of the Klebsiella infection and protects the immune system from excessive antigenic action.