Fifteen influenza-naive Welsh mountain ponies were randomly assigned to three groups of five. A single dose of a recombinant ALVAC vaccine was administered intramuscularly to five of the ponies, two doses, administered five weeks apart, were administered to five, and the other five served as unvaccinated, challenge controls. Two weeks after the completion of the vaccination programme, the ponies were all challenged by exposure to an aerosol of influenza virus A/eq/Newmarket/5/03. Their clinical signs were scored daily for 14 days according to a standardised scoring protocol, and nasal swabs were taken daily for 10 days to monitor the excretion of virus. The challenge produced severe clinical signs of influenza (fever, coughing, nasal discharge and dyspnoea) in all five control ponies, but the vaccinated ponies developed only mild disease, consisting of a serous nasal discharge lasting for only one day. The excretion of virus was almost completely suppressed in the vaccinated ponies, but the control ponies shed the virus for up to seven days after the challenge.