The use of enzymes as immunochemical labels, specifically the direct substitution of enzymes for radionuclides in competitive binding assays as first described in 1971 has been cited by many authors as having certain advantages over corresponding radioimmunoassay techniques. Certain enzyme-immunoassays partially eliminate the need for expensive and sophisticated instrumentation. Some enzyme-immunoassays are read by a visual endpoint and others utilize simple spectrophotometric procedures, while the more sophisticated enzyme-immunoassay procedures require enzyme rate-analyzer capabilities. Enzymes are biological catalysts composed of protein and, in many cases, other organic or inorganic constitutents which facilitate the catalytic activity by direct participation in the chemical reaction or by contributing to the structural integrity of the enzyme. It is a fairly simple matter to quantitate enzymatic activity in simple aqueous-buffer systems. This is not necessarily the case in the dilute solutions of human serum or plasma in which enzyme-immunoassays are usually carried out.