In enzyme immunoassay, the catalytic activity is determined by photometric assay of the substrate, and in a broad sense, it also includes fluorometric and chemiluminescent assays. Since enzyme immunoassay excells in specificity and detectability, it is widely applied to clinical tests which analyze various specimens. Here, we describe some of enzyme amplification methods including cyclic reactions, avidin-biotin reactions and enhanced chemiluminescent enzyme immunoassays. These methods have resulted in highly sensitive immunoassays, the level of which ranges from a attomole (1 x 10(-18) mol) to a zeptomole (1 x 10(-21) mol).