The effects of yogurt containing viable Lactobacillus strain GG (L. GG) and/or fiber supplements on fecal enzyme activities (beta-glucuronidase, nitroreductase, beta-glucosidase, glycocholic acid hydrolase, urease) and on bacterial metabolites in urine (phenol, p-cresol) were studied in 64 females, 20-41 y old. The subjects were randomly divided into three groups: the first group received L. GG yogurt (2 x 150 mL/d, containing 10(11) colony-forming units (cfu)/L of L. GG), the second group received L. GG yogurt and a rye fiber product (30 g/d, equivalent to 9 g fiber/d), and the third group received placebo yogurt (pasteurized) and fiber. The supplementation period lasted 4 wk, with a preceding 2-wk baseline period and a 2-wk follow-up period. The mean fecal count of L. GG was approximately 10(6) cfu/g feces during the supplementation, and L. GG persisted in the fecal samples of 28% of the subjects for 2 wk after supplementation. L. GG yogurt alone or with fiber significantly decreased fecal beta-glucuronidase, nitroreductase and glycocholic acid hydrolase activities. These enzyme activities returned to baseline levels during the follow-up period. beta-Glucosidase and urease activities were not altered significantly during the study. The addition of fiber to L. GG and placebo yogurt had no effect on the enzymic activities. Urinary excretion of p-cresol decreased significantly in groups receiving L. GG. These data demonstrate that L. GG can modify the colonic environment with possible health effects.