A newly isolated strain of a species of Lactobacillus of human origin, designated GG (Lactobacillus GG), has been studied to determine its ability to survive in the human gastrointestinal tract. When fed to 76 volunteers as a frozen concentrate or as a fermented preparation in milk or whey, Lactobacillus GG was recovered in the feces of all subjects receiving the fermented milk or whey and in 86% receiving the frozen concentrate when a single fecal specimen was cultured. The organism was also present in the feces of subjects concurrently receiving ampicillin. After terminating feeding of the organism, Lactobacillus GG persisted in the feces of 87% of volunteers four days later and in 33% of subjects seven days later. Lactobacillus GG lowered fecal bacterial beta-glucuronidase activity by approximately 80% in volunteers given the organism for four weeks. These studies demonstrate that Lactobacillus GG can survive and temporarily colonize the human gastrointestinal tract and can affect the metabolic activity of the resident microflora.