The use of a concentrate containing milk immunoglobulins prepared from rotavirushyperimmunized cows (neutralization titer, 1:6,000 for a 10070 solution) to treat infants hospitalized for acute rotavirus gastroenteritis resulted in a significant (P = . (08) 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 30J0 of cases (neutralization titers, <1:20). Immunoelectrophoresis of stool extracts revealed fragment A, a bovine analogue of Ftab'), 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.