''This study was designed to examine the relationship of waterborne enteroviruses to infections and disease. Young weanling swine and their homologous enteroviruses were chosen as the model system: The porcine digestive tract is like that of man, but pigs can be handled under more closely standardized conditions than humans or other primates. Porcine enteroviruses resemble those of man in every way, but they infect swine so specifically that handling the most virulent of the porcine agents is apparently no threat to the health of research personnel. Known quantities (as measured by the plaque technique in tissue cultures) of two enteroviruses were administered in 5 ml of drinking water in such a way that the subjects were obliged to swallow all of it. The host's body was found to be about 1000 times (600-750 for one virus and 1800-2500 for the other) less likely than the tissue cultures to be infected by a given quantity of enterovirus. The ratio did not depend on whether the animals were fed just before challenge. The probability of infection was cumulative with iterated small doses: this indicated that there was, in the strict sense, no minimum infectious dose. None of the infected animals became ill, despite the reported virulence of the challenge viruses. Chlorine treatment of a concentrated virus suspension, which reduced infectivity to a level detectable by cytopathic effect but not plaque formation in tissue culture, left enough virus to infect one of five challenged subjects. Neither of two colostrum-deprived pigs, challenged by stomach tube with 20 plaque-forming units of enterovirus at 1 h of age, became infected.''