Persistently infected animals (PI animals), that is those animals born after an intrauterine infection of the dam during the first 120 days of gestation, are the main source of bovine virus diarrhoea virus (BVD virus) in a cattle population. The success of any BVD virus eradication programme depends on the ability to detect all PI animals at a young age. The purpose of this study was to evaluate the use of the antigen ELISA test and the reverse transcriptase-polymerase chain reaction (RT-PCR) test for the diagnosis of PI animals in the presence of maternal antibodies, and to compare them with the classical virus isolation test. In this experiment, 25 calves born after an experimental infection with a mixture of BVD virus field strains were used. All calves were found to be positive for BVD virus using the virus isolation test, both before the ingestion of colostrum and again at 10 weeks of age. Both the virus isolation test and the antigen ELISA test were shown to be unreliable indicators for the diagnosis of persistent infections with BVD virus, when used in the presence of high levels of maternal antibodies. However, the RT-PCR test gave positive results even in the presence of high maternal antibody titres, indicating the suitability of the RT-PCR test for use in eradication programmes.