Lactobacillus paracasei KW3110 is a strain which has been reported to possess a strong antiallergic activity. Since many lactic acid bacteria have shown to be useful as the probiotics, the possibility of this strain for use as a probiotic was studied. An in vitro test showed that this strain is strong enough to survive in the gastric juice. In addition, this strain showed exceptionally strong adherence to human intestinal epithelial cells, Caco-2 and HT29 compared to normal L. paracasei strains. Human oral administration testing of L. paracasei KW3110 showed that this strain survived in the human gut and increased the number of bifidobacteria and lactobacilli in the fecal samples of the subjects. In addition, KW 3110 was detected in 50% of the subjects up to 1 week after the end of the administration test. These results showed that L. paracasei KW3110 has a strong ability to survive in and colonize the human gut and improve the human intestinal microflora.