To reproduce experimentally clinical bovine respiratory syncytial virus (BRSV) infections in cattle, we isolated BRSV from a calf in the field that suffered from acute respiratory disease. Cell culture passage of the virus was avoided to prevent any modification of the biological properties of the virus. The isolated BRSV was passaged in specific-pathogen-free (SPF) calves. Lung lavage fluids of these calves, which contained at least 10(3) TCID50/ml BRSV and which were found to be free of other known respiratory pathogens, were collected and pooled for experimental infection. To reproduce a clinical BRSV infection, two groups of six SPF calves were inoculated intranasally with 2 ml of 10(3.9) TCID50/ml BRSV of the obtained virus stock. Another five calves, which were persistently infected with bovine virus diarrhoea virus (BVDV), were given the same inoculum. One group of six calves served as mock-infected controls. Clinical signs were closely monitored from 1 week before until 16 days after inoculation. Reproducible clinical signs consisting of significantly (p < 0.05) increased respiratory rates and elevated body temperatures were recorded but not in all BRSV-inoculated calves. Although clinical signs were induced by experimental infection with non-cell-culture-passaged BRSV, the respiratory signs were not as serious as in the most severe cases in the field.