The basis of a test which is widely used in food microbiology to identify, and quantify the numbers of, microorganisms. The test is based on the reaction of an antigen in the microorganism with an antibody specific to that antigen in the presence of an enzyme which is usually coupled with the antibody. The enzyme is involved in a reaction which has a coloured end-product, and the development of the colour, and its intensity, indicates the presence of the organism and its numbers. A wide variety of commercially produced kits are available to test for microorganisms and toxins in fish products.