The lamb model was used to investigate the possible protective effects of vaccination with inactivated viral antigens against experimental infection with bovine respiratory syncytial virus. Two groups of eight lambs were vaccinated with either glutaraldehyde-inactivated inactivated cell-associated virus or heat-inactivated cell-free virus and subsequently challenged with live virus, along with a group of naive lambs, The virus was shed for significantly longer periods, and the virus titres in nasal secretions were significantly higher in the group of naive lambs than in the two groups of vaccinated lambs. The period of virus-shedding in nasal secretions and virus titres was significantly lower (p <0.01) in the group of lambs immunized with the cell-associated preparation, The same antigen stimulated better cellular immune responses as measured by virus-specific cytotoxicity or by virus-specific lymphocyte proliferation. However; priming with inactivated vaccines had no significant effect on lymphocyte responses to phytohaemagglutinin, which was found to be significantly reduced (p <0.01) following challenge with live virus. (C) 1998 Elsevier Science Ltd. All rights reserved.