The effect of passive immunotherapy with antisera against heat-killed Pseudomonas aeruginosa and three of its exo-enzymes (elastase, alkaline protease and exotoxin A) in gut-derived P. aeruginosa sepsis was evaluated. Mice were given a suspension of P. aeruginosa strain D4 in their drinking water, together with ampicillin (200 mg/kg) to disrupt the normal bacterial flora. Cyclophosphamide was then administered to induce translocation of P. aeruginosa that had colonised the gastrointestinal tract so that gut-derived septicaemia was produced. In this model, intraperitoneal administration of antiserum against heat-killed bacteria, 100 microl/mouse, twice a day for 3 consecutive days significantly increased the survival rate over that of mice treated with normal rabbit serum. Antiserum against elastase, alkaline protease, or a combination of these two antisera, failed to provide significant protection. In contrast, antiserum against exotoxin A significantly increased the survival rate over that of mice treated with normal rabbit serum. These results indicate that passive immunisation with antiserum against heat-killed bacteria and exotoxin A, but not with antiserum against either elastase or alkaline protease, protects mice against gut-derived sepsis caused by P. aeruginosa.