Klebsiella pneumoniae KP1-0 capsular polysaccharide (PS) was isolated from culture supernatants by coprecipitation with N-cetyl-N,N,N-trimethylammonium bromide. PS was composed primarily of carbohydrate and contained less than 1% (wt/wt) protein and nucleic acids. The protective capacity of passively transferred anti-PS was evaluated in a murine burn wound sepsis model. Anti-PS antibody was found to offer high levels of protection (P less than or equal to 0.02) against a low challenge dose of K. pneumoniae KP1-0. The 50% lethal dose for mice which received anti-PS antibody was increased from 10- to 40-fold over that for mice which received normal rabbit serum. Anti-PS antibody was found to be most effective at reducing mortality when administered before the challenge. In experiments designed to monitor the course of the infection, anti-PS was found to both delay the onset of bacteremia and reduce bacterial counts in the blood. Optimal protection against fatal burn wound sepsis was obtained by the use of a combined antibiotic and passive antibody therapy regimen.