Capsular polysaccharide (CPS) from 18 Klebsiella strains of different capsular types was isolated and characterized. Purified CPSs were composed primarily of carbohydrate with trace quantities of protein, nucleic acids, and lipopolysaccharide. All CPSs were of a high molecular weight, possessing a Kd of 0.01 to 0.11 as determined by gel filtration over Sepharose CL-4B. Low levels of lipopolysaccharide present in all preparations were responsible for the highly pyrogenic nature of one-half of the CPS preparations. Treatment of capsular material with dilute NaOH in 95% ethanol markedly reduced the pyrogenicity of all preparations and had a negligible effect on their molecular weight. The immunogenicity of the various native CPSs for mice varied considerably from serotype to serotype, but all evoked an anticapsular immunoglobulin G response. Five of 18 NaOH-treated polysaccharides were significantly (P less than 0.05) less immunogenic than their native counterparts. Human immunoglobulin G prepared from volunteers immunized with either native or NaOH-treated KP1-0 capsular polysaccharide was equally effective at preventing experimental fatal Klebsiella pneumoniae burn wound sepsis in mice.