Bovine coronavirus (BCV) is associated with infection of the enteric and respiratory tracts of neonatal calves, and is a cause of neonatal calf diarrhoea. The virus has 4 major structural proteins: the integral membrane glycoproteins (gps) (M), the nucleo-capsid proteins (N), the spike gps (S) and the haemagglutinin-esterase gps (HE). Faecal samples from diarrhoeic calves were tested for BCV using an enzyme linked immunosorbent assay (ELISA), and attempts made to isolate the virus from positive samples in tracheal organ culture (TOC). Growth of the virus was monitored by measurement of haemagglutination (HA) titres, and confirmed by ELISA. Two out of 17 samples (12%) and 29 out of 60 samples (48%) grew to HA titres of at least 16 in TOCs obtained from bovine foetuses and young calves respectively. Seven out of 12 viruses (58%) isolated in calf TOCs were successfully adapted to growth in human rectal tumour (HRT-18) cells (MRI BCV isolates). Eight monoclonal antibodies (MAbs) were raised against S2 strain BCV (S2 MAbs) and a further 4 MAbs were supplied from the Central Veterinary Laboratory (CVL MAbs). The isotypes of the MAbs were determined, and their protein specificities investigated by Western blotting. One MAb was directed against M, 3 against N, 3 against S and 5 against HE. The MAbs were also characterised in terms of their reactions with S2 viruses in immunofluorescence (IF), neutralisation (SN) and haemagglutination inhibition (HAI) tests. Polyclonal sera and MAbs were used to probe S2 virus proteins in Western blotting experiments. The MWs of the HE, S, N and M proteins were found to be 116 (reducible to 64), 98, 52 and 21 (range 19-23) kD respectively. The S2 MAbs were used in competition ELISAs: the 4 HE MAbs defined a single antigenic region whilst the 3 N MAbs defined 3 distinct regions.