The use of a concentrate containing milk immunoglobulins prepared from rotavirus-hyperimmunized cows (neutralization titer, 1:6,000 for a 10% solution) to treat infants hospitalized for acute rotavirus gastroenteritis resulted in a significant (P = .008) reduction in the duration of excretion of virus. Stool samples from treated infants showed the presence of bovine milk immunoglobulins in 47% of cases and of neutralizing activity in 43% (mean neutralization titer, 1:48); stool samples from control infants showed neutralizing activities in only 3% of cases (neutralization titers, less than 1:20). Immunoelectrophoresis of stool extracts revealed fragment A, a bovine analogue of F(ab')2 or Fab, as the major product of in vitro and in vivo digestion of the immunoglobulins. Cessation of excretion of virus correlated with the appearance of neutralizing activities in 19 of 25 infants. Only concentrate-treated infants with high neutralizing activity in stools showed a statistically significant reduction in duration of excretion of virus; this duration in concentrate-treated infants with low neutralizing activity was comparable with controls.