The effect of orally administered lactobacilli on acute rotavirus diarrhea was tested in 42 well-nourished children ages 5–28 months. After oral rehydration, the patients were randomized to a study group, receiving humanLactobacillus casei strain GG 10¹⁰ colony-forming units twice daily for five days, or a control group not given lactobacilli.Lactobacillus GG was found in the feces in 83% of the study group. The diarrheal phase was shortened in that group. Dietary supplementation with lactobacilli significantly influenced the bacterial enzyme profile: urease activity during diarrhea transiently increased in the control group but not in the study group;F=8.6,P=0.01. No intergroup differences were found in -glucuronidase, -glucosidase, and glycocholic acid hydrolase levels. We suggest that rotavirus infection gives rise to biphasic diarrhea, the first phase being an osmotic diarrhea and the second associated with overgrowth of specifically urease-producing bacteria. Oral bacteriotherapy appears a promising means to counteract the disturbed microbial balance.