The attenuation and immunogenicity of two novel Salmonella vaccine strains, Salmonella enterica serovar Typhi (Ty2 ⌬aroC ⌬ssaV, designated ZH9) and S. enterica serovar Typhimurium (TML ⌬aroC ⌬ssaV, designated WT05), were evaluated after their oral administration to volunteers as single escalating doses of 10 7 , 10 8 , or 10 9 CFU. ZH9 was well tolerated, not detected in blood, nor persistently excreted in stool. Six of nine volunteers elicited anti-serovar Typhi lipopolysaccharide (LPS) immunoglobulin A (IgA) antibody-secreting cell (ASC) responses, with three of three vaccinees receiving 10 8 and two of three receiving 10 9 CFU which elicited high-titer LPS-specific serum IgG. WT05 was also well tolerated with no diarrhea, although the administration of 10 8 and 10 9 CFU resulted in shedding in stools for up to 23 days. Only volunteers immunized with 10 9 CFU of WT05 mounted detectable serovar Typhimurium LPS-specific ASC responses and serum antibody responses were variable. These data indicate that mutations in type III secretion systems may provide a route to the development of live vaccines in humans and highlight significant differences in the potential use of serovars Typhimurium and Typhi.