An ISCOM subunit vaccine was prepared by adsorption of purified viral membrane proteins of BHV-1 on the glycoside Quil A and assayed in six-month-old seronegative calves. Groups of five animals were given three intramuscular doses of 50 or 25 micrograms BHV-1/ISCOMs or a commercial attenuated vaccine. Both types of vaccine induced seroconversion: the ISCOM vaccine consistently gave a serological response superior to that of the attenuated vaccine with maximal titres of 1/608 in haemagglutination inhibition and 1/53 in neutralization for the 50 micrograms dose compared to 1/152 haemagglutination units and 1/16 neutralizing titres for the attenuated vaccine. Calves vaccinated with the ISCOM vaccine were protected upon challenge whereas control animals showed signs of respiratory distress, and calves vaccinated with the attenuated vaccine developed only mild respiratory tract infection and mild increased rectal temperature. Virus shedding was reduced 100-fold in the attenuated vaccine group compared to 10,000-fold reduction in the ISCOM vaccine groups. The high level of protection induced indicates the potential of BHV-1/ISCOM as a subunit vaccine.