After intranasal inoculation of SPF pigs with the 7 reference strains of Haemophilus parasuis only the reference strains of serotypes 1 and 5 proved to be pathogenic and were able to produce the lesions characteristic of Glässer's disease. Even though the reference strains of serotypes 2, 3, 4, 6 and 7 proved to be apathogenic, intranasal inoculation resulted in the development of circulating antibodies as demonstrated by the complement fixation test. This serological response suggests an antigenic relationship between serotypes 1 and 3 different from the relationship seen among serotypes 2, 4, 5, 6 and 7. Pigs given an aerosol with the apathogenic serotypes 2, 3, 4 and 7, respectively, resisted challenge with a virulent Danish strain of serotype 5. These results indicate that the 7 serotypes also have antigenic determinants in common and that these determinants elicit cross-protecting immunity on the respiratory mucosa.