Specific pathogen-free calves were inoculated intranasally with one of seven strains of bovine herpesvirus 1.1 (BHVl.1) to identify a highly virulent strain for use in vaccinationchallenge experiments. The calves were monitored clinically and virologically. Clear differences in virulence between the strains were observed. The Iowa strain was the most virulent; the four calves infected with the strain had the most severe clinical signs; two of them died and viraemia was detected in three of them. To evaluate the immunogenicity of the seven strains all the calves were challenged 16 weeks later with the Iowa strain. The calves of a control group showed the typical signs of a BHV1 infection, whereas all the other calves were protected against disease and shed little or no virus. Hence, the differences in virulence were not associated with differences in immunogenicity. After the calves had been treated with dexamethasone, differences were observed between the strains in the amount of virus that was excreted.