Seven previously untreated five-month-old New Forest ponies received two doses of equine influenza immunostimulating complex vaccines, one with and one without an immunopurified tetanus toxoid component, given by deep intramuscular injection six weeks apart, followed by a booster dose without tetanus toxoid five months later. Fifteen months after the third dose of vaccine, the ponies were challenged by exposure to an aerosol of influenza A/Equine 2/Sussex/89 (H3N8), a virus isolated from a recent outbreak of influenza A/equine 2 in Britain. The challenge produced severe clinical signs of influenza (pyrexia and coughing) in five unvaccinated control ponies. Four of the vaccinated ponies were completely protected against clinical disease, and two of these were also protected against infection as demonstrated by their lack of an antibody response after challenge. No coughing was recorded among the vaccinated ponies, and only three of the seven vaccinated ponies experienced a transient mild pyrexia. The mean duration and severity of the pyrexia among the vaccinated ponies was significantly less (P < 0.01) than among the controls, and the excretion of virus was almost eliminated, thus demonstrating the protective efficacy of the vaccines 15 months after vaccination. Monitoring of tetanus antitoxin antibodies showed that protective levels (> or = 0.01/iu/ml) were maintained for at least 20 months after vaccination.