Guinea pigs were fed L. pneumophila through an orogastric tube. Gastric acid was rapidly cidal to the organisms. Serial necropsies demonstrated the organisms in the colon and blood stream at 1 hr. Guinea pigs fed large doses of L. pneumophila seroconverted. Previously fed guinea pigs were then challenged with a lethal intraperitoneal dose of L. pneumophila and were protected in a dose-dependent fashion. Pretreatment of the guinea pigs with cimetidine lowered the dose of oral L. pneumophila required for protection. This model may be useful in studying the immune response after oral ingestion of L. pneumophila.