The objective of this study was to evaluate the efficacy of a live, attenuated bovine (strain WC3) x human (strain WI79, serotype G1) rotavirus reassortant (WI79-9) virus vaccine for prevention of symptomatic rotavirus gastroenteritis in infants. The study was a prospective, randomized, double-blind, placebo-controlled trial, conducted over a single rotavirus season in 325 infants who were 2 to 8 months old at enrollment. Subjects were randomized to receive either placebo or WI79-9 virus vaccine at 10(7.3) plaque-forming units in three oral doses each separated by 2 months. Subjects were followed for 7 days after each dose for occurrence of adverse events and during the subsequent winter for development of rotavirus gastroenteritis. Administration of WI79-9 virus vaccine was well-tolerated, and the rates of low grade fever after each dose were no higher in vaccine recipients (8 to 21%) than in placebo recipients (14 to 19%). The protective efficacy of the WI79-9 vaccine during a subsequent epidemic of predominantly serotype G1 rotavirus was 87.0% (95% confidence limits, 62.6 to 95.5%) against relatively severe rotavirus gastroenteritis (rotavirus gastroenteritis with a clinical severity score of > 8) and was 64.1% (95% confidence limits 35.9 to 79.9%) against all symptomatic rotavirus episodes. The WI79-9 vaccine was safe and effective in prevention of homotypic human rotavirus infection in infants. Further studies of reassortant vaccines based on the bovine WC3 rotavirus should be performed.