The colonization of Lactobacillus rhamnosus GG (ATCC 53103, henceforth L.GG) in five human colonoscopy patients was studied. The test subjects consumed whey drink fermented with the bacterium for 12 d before the colonoscopy. The presence of L.GG was subsequently checked both in the faecal samples and in the colonic biopsies obtained from various locations in the large intestine. In all patients L.GG was the dominant faecal lactic acid bacterium as a result of the administration. In four patients L.GG could also be recovered from the biopsies, while with one patient (suffering from ulcerative colitis diagnosed during the colonoscopy) no L.GG was detected in the biopsy samples. The results suggest that L.GG is able to adhere in vivo to the colon. Study of the faecal samples alone is apparently not sufficient for elucidation of the gastrointestinal ecology of probiotic bacteria.