Four groups of pigs immunized with different vaccines and a group of non-vaccinated controls were challenged intratracheally with a lethal dose (5 x 10(9) colony-forming units) of Haemophilus parasuis, the aetiological agent of Glässer's disease. A vaccine containing inactivated whole organisms gave strong protection against clinical signs, death, pathological changes and persistence of organisms in vivo. However, all non-immunized pigs, all pigs given a vaccine consisting of the recombinant transferring-binding protein (Tbp) B, some pigs given an outer membrane protein (OMP) formulation enriched with TbpB and some pigs immunized with a sub-lethal dose of live organisms died at various times after challenge, yielding positive cultures from most organs post mortem and having shown hyperthermia and other clinical signs before death. Animals that died showed fibrinosuppurative polyserositis, exudative pneumonia, and lesions compatible with acute septicaemia, e.g., disseminated intravascular coagulation with multiple fibrinous thrombi in arterioles and capillaries, depletion of splenic white pulp, and acute lymphadenitis. The results suggested that, in addition to the protection given by inactivated whole organisms, partial protection was given by the OMP formulation and by a sub-lethal dose of living organisms; however, the recombinant TbpB preparation gave no protection.