Three types of yogurt supplemented with Bifidobacterium alone, lactulose alone, or both Bifidobacterium and lactulose were fed to healthy persons, who were tested before and after administration. Plain yogurt was fed during a control period. Yogurt containing both Bifidobacterium and lactulose resulted in greater improvement in the form and frequency of feces than the other yogurts. No differences were noted in blood chemistry test results before and after the administration of each type of yogurt. There were no side effects of yogurt administration. The number of Bifidobacterium in the feces increased after the administration of yogurt containing Bifidobacterium and/or lactulose, but there were no differences among the three test yogurts. Not only the administered Bifidobacterium species but also the original Bifidobacterium species was increased by eating Bifidobacterium and lactulose-containing yogurt, but the administered Bifidobacterium species was not increased more than the dominant original species. The ammonia content in the feces decreased more after the administration of Bifidobacterium-and/or lactulose-containing yogurt than after that of plain yogurt but there were no differences among the three test yogurts.