A dried decoction of a kampo (Japanese herbal) medicine, Juzen-Taiho-To (TJ-48), has been fractionated into five fractions and tested for their effects on antibody response of mice. An intraperitoneal injection (300 mg/kg) of TJ-48 stimulated anti-sheep red blood cell (SRBC) antibody response of normal Balb/c mice, but only the polysaccharide fraction (F-5, 300 mg/kg) enhanced the antibody response among the fractions from TJ-48. When F-5 (0.5 or 1.0 g/kg/day) was orally administered to normal Balb/c mice (7-weeks-old) from 7 days before to 4 days after immunization with SRBC, the number of anti-SRBC-IgM-PFC in spleen and the titer of anti-SRBC-IgM in plasma were increased significantly. However, a lower dose (0.1 g/kg/day) of F-5 did not show a significant stimulative activity on the anti-SRBC-response. Although aged Balb/c mice (6-months-old) produced a lower level of anti-SRBC-IgG in comparison with young Balb/c mice (8-weeks-old), the anti-SRBC-IgG response of the aged mice was stimulated significantly when F-5 (0.13 g/kg/day) or TJ-48 (1.0 g/kg/day) was orally administered to the aged mice from 6 days before immunization. Intraperitoneal injections of i-carrageenan (2.5 mg/kg/day) at 3 and 1 days before the immunization with SRBC increased the level of anti-SRBC antibody response compared with normal mice. Oral administrations of TJ-48 (1.0 g/kg/day) or F-5 (0.5 g/kg/day) to the i-carageenan-treated mice reduced the level of the anti-SRBC-antibody response near to that of normal mice.(ABSTRACT TRUNCATED AT 250 WORDS)