The occurrence of an outbreak of Pseudomonas aeruginosa septicaemias on an oncology ward permitted an analysis of antibody responses in patients who were all orally exposed to the same source of infection. Seven patients became septicaemic. Serial serum samples were immunoblotted against the homologous strain. Responses were compared with those of 16 other patients with septicaemias caused by other strains and 10 healthy controls. All 18 survivors produced increasing IgG or IgA antibody, or both, against a 35,000 dalton band, whereas these antibodies were usually absent or fell in titre in the five fatal cases. These antibodies were also lacking in sera taken just before a patient became septicaemic. This band had the electrophoretic characteristics of the outer membrane porin protein F.