A serological survey was carried out on pig sera from herds in Northern Ireland to investigate the incidence of reactivity to bovine respiratory syncytial virus (BRsv) antigens. A total of 529 pig sera from 61 herds were tested and 219 (41 per cent) were found to be reactive with BRsv-infected cell cultures in an indirect immunofluorescence test. None of the BRSV-reactive sera immunostained turkey rhinotracheitis virus-infected cell cultures, indicating specificity for BRsv epitopes. The specificity of this reactivity for BRSv antigen was confirmed by double immunolabelling, using monoclonal antibodies to BRSV and two pig sera with different reactivities to BRSv antigens. A longitudinal serological investigation of two litters of pigs indicated that BRSV-serum reactivity developed between six and 11 weeks after birth. The immunofluorescent staining pattern observed with the majority (73 per cent) of the BRsv-reactive pig sera was typical of that observed with known BRsv-reactive bovine sera. The other immunoreactive pig sera stained BRsv-infected cell cultures in an atypical staining pattern. These different reactivity patterns, combined with the results of the serum neutralisation tests, suggest that more than one serotype of a porcine pneumovirus may exist.