1,2-Dihydroxyanthraquinone (1) was isolated from the seed of Cassia obtusifolia through bioassay-guided fractionation. 1,2-Dihydroxyanthraquinone strongly inhibited the growth of Clostridium perfringens and Escherichia coli. Structure-activity relationship revealed that 1,4-dihydroxyanthraquinone (2) and 1,8-dihydroxyanthraquinone (3) had strong growth-inhibition against C. perfringens. In growth-promoting activity, 1,2-, 1,4-, and 1,8-dihydroxyanthraquinones exhibited strong growth-promoting activity to Bifidobacterium bifidum.