A human stool filtrate containing rotavirus which was administered orally to gnotobiotic lambs caused diarrhea, virus excretion, development of antibodies to rotavirus, and pathological changes in the villi of the small intestine. Thus, lambs may serve as experimental animals for the study of human rotavirus infections. This model system was used to study passive protection. Human immunoglobulin G (IgG) containing antibody to rotavirus was fed to lambs 24 to 78 h after birth, and the lambs were infected with lamb-passaged human rotavirus when 30 h old. The lambs treated with IgG did not develop diarrhea, and virus excretion was delayed in onset and shortened in duration. It may be possible to make similar use of IgG to protect children at risk in a rotavirus outbreak. The treatment did not prevent the lambs developing antibody to rotavirus.