The ability of Lactococcus strains to inhibit the growth of intestinal bacteria was examined. In in vitro cocultures, we observed that among eighteen Lactococcus strains tested, the ability to inhibit growth of Escherichia coli varied, with the L. lactis N7 showing the greatest growth inhibition. Strain N7 ($8.94 imes10^{10}$ CFU/day for 7 days) was orally administered to mice, and the viable count of strain N7 in feces appeared at a level of $10^{4-5}$ CFU/g. After administration, the proportion of Bacteroidaceae to total intestinal bacteria decreased. Lactococci may act as probiotic bacteria by inhibiting the growth of harmful bacteria.