Three hundred twenty-two patients with injuries involving burns over 20% of their total body surface areas have been actively immunized with a polyvalent vaccine against Pseudomonas aeruginosa at the University of Cincinnati Burn and Trauma Unit and the Cincinnati Shriners Burns Institute during the five-year period preceding April 1973. Of a similar group of 75 consecutive unvaccinated controls treated between March 1967 and April 1968, 14.7% died from pseudomonas infection, and 18.7% died from pseudomonas bacteremia. With an increase in dose and in duration of vaccination, the incidence of pseudomonas bacteremia declined to 6%. Of 96 patients who were immunized with the maximal dose, 3.1% died, but none of the last 186 vaccinated pateints, for whom hyperimmune γ-globulin to Pseudomonas was used as adjunctive treatment of pseudomonas bacteremia, has died. These clinical observations have been accompanied by a variety of immunologic studies, which confirmed that immunization with the polyvalent vaccine induced production of an immune IgG antibody. Invasive infection with P. aeruginosa is accompanied or preceded by a variety of immunologic deficiencies that can be compensated for by the administration of immune IgG antibody. Predominant among these deficiencies are abnormalites of the alternate pathway of complement and of the antibacterial function of neutrophils. Our results strongly suggest that patients with major thermal injuries should be actively vaccinated against Pseudomonas, and that hyperimmune γ-globulin to Pseudomonas is an effective adjunctive measure for the treatment of life-threatening infections caused by P. aeruginosa.