The theoretic basis for developing conjugate vaccines, to induce immunoglobulin G (IgG) lipopolysaccharide (LPS) antibodies for the prevention of shigellosis, has been described (J. B. Robbins, C.-Y. Chu, and R. Schneerson, Clin. Infect. Dis. 15:346-361, 1992). The 0-specific polysaccharides (0-SPs) ofShigeUa dysenteriae type 1, S. flexneri type 2a, and S. sonnei were covalently bound to carrier proteins. Alone, the O-SPs were not immunogenic in mice. Conjugates of these O-SPs, injected into young outbred mice subcutaneously as saline solutions containing 2.5 ,ug of saccharide, elicited serum IgG and IgM antibodies with booster responses; adsorption onto alum enhanced their immunogenicity. Injection of 25 ,ug of these conjugates into adult volunteers elicited mild local reactions only. Each conjugate induced a significant rise of the geometric mean serum IgG, IgM, and IgA LPS antibody levels. A second injection 6 weeks later did not elicit booster responses, and adsorption of the conjugates onto alum did not enhance their immunogenicity. Conjugate-induced levels of IgA, but not IgG or IgM, declined to preimmunization levels at day 56. The levels of postimmunization antibodies of the three immunoglobulin classes were similar to or higher than those of recruits in the Israel Defense Force following shigellosis caused by S. flexneri type 2a or S. sonnei. These data provide the basis for evaluating these conjugates to prevent shigellosis. * Corresponding author. composed of the 0-specific polysaccharides (O-SPs) of the three most common serotypes of shigellae, namely, Shigella dysenteriae type 1 (Shiga's bacillus), S. flexneri type 2a, and S. sonnei, has been reviewed (32, 57). We suggested how serum antibodies could exert protective immunity to enteric pathogens (25, 41). First, the inoculum that survives the gastric acid is probably only-102 shigellae. Second, serum immunoglobulin G (IgG) antibodies (and probably complement) exude into the intestinal lumen and are kept in contact with the contents of the lumen by peristalsis (39). Third, shigellae have short O-SPs which are associated with susceptibility to serum antibody plus complement. These factors could lyse shigellae on the intestinal surface. The synthesis and immunologic properties in mice of an S. dysenteriae type 1 conjugate have been reported (4). In this report, we describe the synthesis of conjugates for S. flexneri type 2a and S. sonnei with the recombinant Pseudomonas aeruginosa exoprotein A (rEPA) and their immunologic properties in mice (3, 17, 18). These three conjugates were evaluated for their safety and immunogenicity in adult volunteers (phase 1). In addition, we compared the levels of lipopolysaccharide (LPS)-specific antibodies elicited by these conjugates with those of adults following shigellosis caused by S. flexneri type 2a and S. sonnei (5-8).