Methodologies for animal disease diagnosis and the analysis and control of animal health currently make much use of immunoassay techniques, in particular ELISA, and tests based on nucleic acid measurement (nucleic acid hybridisation, polymerase chain reaction [PCR]). In the latter area, the recently developed PCRs have become most useful. Recombinant technologies have yielded defined and extremely pure antigens, which can give more accurate interpretations of results obtained using diagnostic tests; in addition, such antigens obviate the need to handle live pathogens, except where these may be contained within the sample for diagnosis. The application of monoclonal antibodies (MAbs) to many tests, in particular those which use immunoassay bases or perform molecular fingerprinting of isolates, has increased the specificity and usefulness of these methodologies. Modern technology has permitted a considerable reduction in the use of animals for MAb generation through in vitro immunisation methods and the application of serum-free media with bio-reactors. These in vitro immunisation procedures have an additional application in the analysis of vaccine efficacy without recourse to animal experimentation. Furthermore, in vitro methods can be used to monitor the immune response in an animal and, in particular, to measure the development of immunological memory. It is now possible to estimate more accurately the quality and duration of an induced immune response without using animals for testing. Not only are the assay systems developed through biotechnology more sensitive and less hazardous than previous techniques (especially with regard to the necessity of handling live pathogens), but they can also determine more accurately the immune responsiveness or immune status of the animals under study.