The genes encoding the enterohemorrhagic Escherichia coli (EHEC) type III secretion system (TTSS) and five effector proteins secreted by the TTSS are located on the locus of enterocyte effacement (LEE) pathogenicity island. Deletion of tir, which encodes one of these effector proteins, results in a profound reduction (∼10,000-fold) in EHEC colonization of the infant rabbit intestine, but the in vivo phenotypes of other LEE genes are unknown. Here, we constructed in-frame deletions in escN, the putative ATPase component of the TTSS, and the genes encoding the four other LEE-encoded effector proteins, EspH, Map, EspF, and EspG, to investigate the contributions of the TTSS and the translocated effector proteins to EHEC pathogenicity in infant rabbits. We found that the TTSS is required for EHEC colonization and attaching and effacing (A/E) lesion formation in the rabbit intestine. Deletion of escN reduced EHEC recovery from the rabbit intestine by ∼10,000-fold. Although EspH, Map, EspF, and EspG were not required for A/E lesion formation in the rabbit intestine or in HeLa cells, these effector proteins promote EHEC colonization. Colonization by the espH and espF mutants was reduced throughout the intestine. In contrast, colonization by the map and espG mutants was reduced only in the small intestine, indicating that Map and EspG have organ-specific effects. EspF appears to down-regulate the host response to EHEC, since we observed increased accumulation of polymorphonuclear leukocytes in the colonic mucosa of rabbits infected with the EHEC espF mutant. Thus, all the known LEE-encoded effector proteins influence EHEC pathogenicity.