Antibody titers against Pseudomonas aeruginosa in sera from mice were markedly elevated by pre-injection of Lactobacillus casei. The number of plaque-forming cells against sheep red blood cells (SRBC) in the spleen of mice pretreated with L. casei was highter than those in the controls. L. casei also enhanced the delayed type hypersensitivity response against SRBC in mice. Therefore, it was revealed that L. casei possesses the actions of humoral and cellular immunopotentiation.