This paper aims to evaluate different forms of the enzyme-linked immunosorbent assays (ELISA) for the detection of virus-specific antibodies and focuses on factors which may influence the diagnostic reliability of such tests. The differences between competitive and non-competitive ELISAs are described, with reference to the influence of the antigen, the conjugated antibody, and the test sample on the test results. Attention is drawn to interference, which may result in false positive or false negative test results. The use of monoclonal antibodies and discriminatory tests is briefly discussed. Diagnostic reliability is described for tests that are used in monitoring or eradication programmes, emphasising the consequences of false negative or false positive results.