Laboratory diagnostic procedures for bovine virus diarrhoea and mucosal disease are reviewed. For serology, emphasis is placed on the virus neutralisation test and enzyme immunoassays. The techniques and the pitfalls which may be encountered in culture procedures for isolation of the virus are described, with special reference to the use of immunofluorescence and immunoperoxidase methods for the detection of non-cytopathogenic virus strains. Viral antigen may also be detected directly in clinical or pathological samples. Monoclonal antibodies are expected to play an increasing role in diagnostic tests, and the application of gene probes for virus detection is anticipated in future years. In the interpretation of diagnostic laboratory test results, the importance is stressed of reviewing them in the context of the herd disease and breeding history, and relating them to current understanding of the pathogenesis and epidemiology of the disease.