This study followed the early pathogenesis of orally induced murine typhoid fever. Intragastrically administered Salmonella enteritidis moves quickly through the normal undisturbed gut so that only a small residuum remains in the cecum and large intestine after the first few hours. Dye injection of the gut wall was used to show that lymph from discrete portions of the gastrointestinal tract drains to separate lymph nodes, probably via the regional Peyer's patches. Plating techniques capable of detecting a single colony-forming unit of S. enteritidis within the different Peyer's patches and draining lymph nodes indicate that, although the cecum and large intestine are exposed to large numbers of Salmonella for longer time periods than the small intestine, the primary site of bacterial penetration involves the distal ileum. This area of the small intestine as well as the cecum are both drained by the distal mesenteric lymph nodes, and were the only nodes which contained detectable numbers of viable Salmonella over the first 24 h of infection. Neither the pyloric nor the proximal mesenteric lymph nodes (which drain the stomach and duodenum) nor the pancreatic and caudal lymph nodes (which drain the transverse and descending colon) contained viable Salmonella . Salmonella were observed to infect the ileal mucosa and its Peyer's patches. With time, this infection progresses to the draining lymph node and ultimately reaches the liver and spleen. Some of the implications of these findings relative to the development of acquired resistance to enteric disease are discussed.