A technique which enables minute quantities of natural substances in the blood such as HORMONES to be measured. It depends on the ability of an unlabelled hormone to inhibit, by simple competition, the binding of isotopically labelled hormone by specific ANTIBODIES. Radioimmunoassay is more sensitive than the best bioassay for a given hormone, and the most sensitive radioimmunoassays permit the detection of picogram (pg = 10–12g) and femtogram (fg = 10–15g) amounts of material.