The effects of drinking a fermented milk beverage that contains Lactobacillus casei strain Shirota (LcS) at 40 billion bacterial cells/bottle for 4 weeks (probiotics, 1 bottle/day) on defecation frequency, intestinal microbiota and the intestinal environment of healthy individuals with soft stools were evaluated. Thirty-four healthy adults who had soft stools were randomised into 2 groups, and the effects of a regular 4-week intake of probiotics were evaluated by a placebo-controlled, double-blind, parallel-group comparative design. Defecation frequency significantly decreased after the 4-week intake period compared with before the probiotic treatment. The stool quality significantly improved (hardened) compared to the placebo. Also, the water content of the stools was lower in the probiotic group than in the placebo group. Live LcS was recovered at 6.9 +/- 1.3 and 7.2 +/-0.8 logio CFU per 1 g of stool after 2 and 4 weeks, respectively, of probiotic treatment. The number of bifidobacteria in the stools also increased significantly compared with the level before starting the probiotics. The organic acid levels (total, acetic acid, propionic acid, and butyric acid) significantly increased compared with the level before intake in both the probiotic and placebo groups, but they returned to the original levels after the end of the intake period. These results suggest that probiotic fermented milk beverage has an intestine-conditioning effect by improving the frequency of defecation and stool quality and increasing the intrinsic bifidobacteria in healthy individuals with soft stool. (C) 2010, The Society for Biotechnology, Japan. All rights reserved.