Pigs inoculated intranasally with Haemophilus parahaemolyticus, serotype 2, resisted challenge 3 weeks later with serotypes 2, 4 and 5 without showing clinical symptoms. The pigs were sacrificed 2 days after challenge, and post mortem examination showed a chronic pleuropneumonia from which only serotype 2 was re-isolated. Pigs inoculated intranasally with H. parahaemolyticus, serotype 2, showed no clinical symptoms when challenged 3 weeks later with serotype 1. Post mortem examination revealed a chronic pleuropneumonia with areas of necrosis from which H. parahaemolyticus, serotype 2, was re-isolated, but also small areas of a more acute fibrinous pneumonia from which serotype 1 was re-isolated. The control pig inoculated with only serotype 1 showed a severe acute fibrinous pleuropneumonia. The results indicate that a considerable cross immunity exists between the various serotypes of H. parahaemolyticus.