Turkeys exposed to Bordetella avium were vaccinated against fowl cholera with live Pasteurella multocida vaccine. Previous exposure to B. avium resulted in impairment of systemic immunity conferred by the vaccine: 86% of the vaccinated turkeys exposed to B. avium at 1 day old developed lesions or died of fowl cholera after challenge at 15 weeks old with virulent P. multocida. Of vaccinated turkeys not previously exposed to B. avium, only 26% had lesions or died of fowl cholera.