Rotavirus infection is an important cause of gastroenteritis in infants and young children. Since the virus replicates in the intestinal lumen, we investigated the presence and effectiveness of rotavirus antibody in three forms of milk: raw milk, pasteurized milk, and commercially available infant formulas. Both raw and pasteurized milk contained detectable levels of IgG1 antibody directed at rotavirus. On the other hand, little or no anti-rotavirus antibody was detected in commercially available infant formulas or other sterile milk preparations. The milk samples with rotavirus antibody were capable of inhibiting the replication of simian, bovine, and human rotaviruses in tissue culture. In addition, they were capable of protecting mice from infection and disease in a murine model of rotavirus infection. On the other hand, the formula preparations were incapable of modifying the in vitro replication of rotavirus strains in tissue culture and did not prevent symptomatic gastroenteritis in the mouse model. We conclude that the alteration of milk-processing procedures or the addition of effective antibodies to milk preparations commonly used in the nutrition of young children may alter the clinical course of rotavirus infection or decrease the transmission of rotavirus throughout susceptible populations.