Sixty-six pigs were vaccinated with formaldehyde-killed 24-hour or 6-hour culture of H. parahaemolyticus with Freund's incomplete adjuvant (1:1 or 3:1) or 15 per cent Alhydrogel. The vaccine was injected subcutaneously when the pigs were 9 weeks old. A booster injection was given 2 weeks later. Two weeks after the booster injection the pigs were challenged by intranasal inoculation of 10(10) organisms. The efficacy of the vaccine was evaluated by clinical observations and autopsy findings. Vaccines with Alhydrogel adjuvant: With 24-hour culture 25 per cent of the pigs were protected. With 6-hour culture the protection reached 67 per cent. Vaccines with Freund's incomplete adjuvant: With 24-hour culture 67 per cent of the vaccinates were protected. With 6-hour culture 90 per cent were protected. It is concluded that a 6-hour culture of H. parahaemolyticus is superior to an older culture in immunizing power. Vaccine with Freund's incomplete adjuvant caused severe local reaction with persistent granuloma formation. For that reason it is questionable whether this vaccine will be accepted for field use. H. parahaemolyticus could be found in the respiratory tract, also of animals that were protected by vaccination. Further studies on the significance and duration of the carrier state in this infection are required.