A phase 1 clinical trial was conducted among 35 healthy adult volunteers to evaluate the safety, immunogenicity, and shedding of different doses of CVD 1207, a live attenuated Shigella flexneri 2a vaccine candidate with specific deletion mutations invirG, sen, set, andguaBA. CVD 1207 retains the ability to invade epithelial cells but cannot effectively spread intercellularly after invasion (ΔvirG), does not produce enterotoxin (Δsenand Δset), and has limited proliferation in vivo (ΔguaBA). In a consecutive fashion, groups of three to seven subjects ingested a single oral dose of CVD 1207 at an inoculum of either 10⁶, 10⁷, 10⁸, 10⁹, or 10¹⁰ CFU. CVD 1207 was remarkably well-tolerated at inocula as high as 10⁸ CFU. In comparison, one of 12 subjects who received 10⁹ CFU experienced mild diarrhea and another experienced a single episode of emesis. One of five subjects who received 10¹⁰ CFU experienced watery diarrhea and emesis. All subjects who ingested doses of 10⁸ to 10¹⁰ CFU excreted the vaccine; in 23 of 25, the duration of excretion was ≤3 days. A dose-related, immunoglobulin A antibody-secreting cell (ASC) response to S. flexneri 2a O-specific lipopolysaccharide was seen, with geometric mean peak values of 6.1 to 35.2 ASCs/10⁶peripheral blood mononuclear cells (PBMC) among recipients of 10⁷ to 10¹⁰ CFU. The cytokine response toShigella-specific antigens observed in volunteers' PBMC following vaccination suggested a Th1 pattern with stimulation of gamma interferon and absence of interleukin 4 (IL-4) or IL-5. CVD 1207 represents a Shigella live oral vaccine strain prepared from wild-type S. flexneri 2a by rational use of recombinant DNA technology that achieves a remarkable degree of attenuation compared with earlier recombinant strains, even when administered at high dosage.