Germ-free mice develop low levels of delayed hypersensitivity following exposure to sensitizing doses of sheep erythrocytes, compared to that seen in conventionally raised mice. Infection of the germ-free animal with either Salmonella gallinarum or Salmonella pullorum was followed by extensive growth of the organisms within the intestine and the gut-associated lymphoid organs. As many as 5,000 viable Salmonellae were recovered from the cecal and ileal Peyer’s patches as well as the mesenteric lymph nodes. However, neither strain spread significantly beyond the lymph nodes to the blood stream or to the liver and spleen. The gnotobiotic mice developed significant levels of delayed hypersensitivity to the Salmonella protein antigen and could be more readily sensitized to sheep erythrocytes than the germ-free host, despite the fact that Salmonellae were only able to infect the gut-associated lymphoid tissues.