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Specialized epithelia known as M cells overlying the lymphoid follicles of Peyer's patches are important in the mucosal immune system, but also provide a portal of entry for pathogens such as Salmonella typhimurium, Mycobacterium bovis, Shigella flexneri, Yersinia enterocolitica and reoviruses^,,,. Penetration of intestinal M cells and epithelial cells by Salmonella typhimurium requires the invasion genes of Salmonella Pathogenicity Island 1 (SPI1)^,,,,,. SPI1-deficient S. typhimurium strains gain access to the spleen following oral administration and cause lethal infection in mice without invading M cells^, or localizing in Peyer's patches, which indicates that Salmonella uses an alternative strategy to disseminate from the gastrointestinal tract. Here we report that Salmonella is transported from the gastrointestinal tract to the bloodstream by CD18-expressing phagocytes, and that CD18-deficient mice are resistant to …