Bovine herpesvirus 1 is the cause of a severe respiratory disease in cattle known as infectious bovine rhinotracheitis (IBR). Restriction endonuclease analysis (REA) of thirteen Irish BHV1 isolates indicates that both BHV1 genotypes (BHV1.1 and BHV1.2) are present here. BHV1.1 has being associated with some of the more severe clinical diseases signs in recent years. A PCR assay to detect BHV1 was developed using oligonucleotide primers chosen from the thymidine kinase (TK) gene. The assay was sensitive (< 1 TCID50) and specific for BHV1 and did not detect the live anti-IBR vaccine Tracherine (SmithKline Beecham). REA of PCR product with the enzymes Hae II and Taq I showed fragments of the expected size. Partial sequence data obtained from the amplification product also correlated with the published sequence data from this region. The assay was also designed to detect BHV5 (formerly known as BHV1.3) which causes fatal encephalitis in young calves. Amplified product from BHV5 could be distinguished from that of BHV1 on the basis of product size and by restriction analysis with the restriction enzyme Taq I. The PCR assay compared favourably to two other routine methods of BHV1 detection (fluorescent antibody test- FAT and virus isolation - VI) when applied to 105 diagnostic submissions from cattle with respiratory disease during a winter period (1992-1993). When used to detect BHV1 in spiked semen samples, the assay detected virus at 1 TCID50 thus offering a very attractive alternative technique to V I . Finally, the study involved the experimental inoculation of a BHV1.1 field isolate (associated with an outbreak of severe pharyngitis in neonatal calves) into calves. Mild clinical IBR infection ensued with little sign of the severe pharyngitis. Using REA isolates remained genotype BHV1.1 after passage in infected animals. PCR detected BHV1 in more nasal secretion samples from infected calves than either VI or FAT.