Field outbreaks of influenza that occurred in vaccinated Thoroughbred racehorses in Newmarket in 1995 and 1996 were investigated by nucleoprotein ELISA and serology. Investigations showed that serum levels of vaccine-induced single radial haemolysis (SRH) antibody correlated closely with protective immunity against equine influenza and were consistent with observations made in previous experimental studies using nebulised aerosol challenge. In the second part of this study, antibody levels stimulated by vaccination were investigated to examine probable protection in high risk groups, such as yearlings and horses in training. Results for yearlings correlated closely with experimentally derived antibody profiles described for several equine influenza vaccines. The horses in training had levels of antibody immediately prior to revaccination, which were higher than those measured in the yearlings. In conclusion, SRH antibody, used in the investigation of outbreaks and surveillance of post vaccination responses, was shown to correlate with and validate experimental vaccination and challenge models currently used in ponies in the licensing of modern vaccines. There may be benefit from serological monitoring of horses following vaccination through identification of susceptible periods to infection and demonstration of poor vaccine responders. This would allow appropriate and timely amendment of vaccination strategies to maximise protective immunity against influenza.