A whole cell formalin killed trivalent Haemophilus parasuis bacterin was tested for efficacy in four week old, weaned specific pathogen free pigs challenged under laboratory conditions. The vaccine contained three field strains of H. parasuis selected from confirmed cases of Glasser's disease. Two different formulations were evaluated in separate trials. In trial 1, ten pigs received 5 mL of bacterin subcutaneously in the neck, followed by a second 5 mL dose two weeks later. Another ten pigs served as nonvaccinated controls. One week after the second dose, all pigs were subjected to an aerosol challenge containing the strains of H. parasuis present in the vaccine. In trial 2, a broth rather than a saline based vaccine was prepared, and tested as in trial 1. In both trials, the vaccinated pigs remained healthy postchallenge, while eight of nine (Trial 1) and eight of ten (Trial 2) nonvaccinated pigs succumbed to Glasser's disease.