Radioimmunoassays are simple and quite useful, being easily applied on a commercially serviceable basis. Furthermore radioimmunoassays are extremely sensitive and quite precise. Two site immunoradiometric assay in which one antibody was immobilized and another labelled with 125I becomes gradually popular, due to its higher sensitivity and the development of monoclonal antibody production by the hybridoma technique. Alternative analytical methods other than radioimmunoassays (radioimmunoassay, immunoradiometric assay) have been developed, in which the alternative labels to substitute for radioisotopes are enzymes, luminescent compounds and fluorescent probes.