The safety and immunogenicity of two Klebsiella pneumoniae K1 capsular polysaccharide (CPS) vaccines were evaluated in humans. Trace quantities of lipopolysaccharide present in vaccine preparations were detoxified by treatment of K1 CPS in a 95% ethanol-0.1 N NaOH solution. This procedure greatly reduced the pyrogenicity of K1 CPS but did not markedly alter its antigenicity, molecular size, or immunogenicity for animals. Volunteers received either 25 or 50 micrograms of untreated or NaOH-treated K1 CPS vaccine subcutaneously. Systemic reactions on primary vaccination were infrequent with both vaccine preparations. However, the frequency and severity of local reactions were substantially reduced after immunization with NaOH-treated vaccine as compared with untreated K1 CPS. All vaccinees responded with a fourfold or greater rise in IgG and IgM titers. IgG antibody to K1 CPS isolated from immune sera was highly effective in preventing fatal experimental burn wound sepsis due to K. pneumoniae K1 in mice.