Since a water-soluble polysaccharide (KGF-C) from the kefir grains was shown to have the property of retarding tumor growth in vivo when administered orally, the effect of KGF-C was examined on antibody responses to thymus-dependent antigen, sheep red blood cells (SRBC), and thymus-independent antigen, dinitrophenyl-Ficoll and trinitrophenyl-lipopolysaccharide. Antibody response in mice intubated with KGF-C was enhanced to low doses of SRBC, but not to optimal or high doses. The optimal dose of KGF-C required for the enhancement was 100 mg/kg body weight. The time-course studies on KGF-C administration implied that KGF-C exerted its effect on the early events of anti-SRBC response. The enhancement was not due to the alteration of kinetics of anti-SRBC responses. Furthermore, the enhancing effect on antibody responses to thymus-independent antigens, such as dinitrophenyl-Ficoll and trinitrophenyl-lipopolysaccharide, was observed neither in nu/nu nor in nu/+ mice, and the effect on delayed-type hypersensitivity response to a low dose of SRBC in normal mice was also found. These findings suggest that the oral immune enhancement by KGF-C is elucidated probably through T-cell but not through B-cell participation.