Enzyme-linked immunoassays (ELISA), has become one of the most popular methods. This chapter modifies the originally described ELISA in laboratory to measure antibodies in subclasses and in minor immunoglobulin classes. It discusses the details of the amplified ELISA (a-ELISA), its comparison to other ELISAs, and experiences in using this assay to measure the distribution of antibodies among isotypes. The chapter presents data on optimal antigen concentrations, the adsorption characteristics of antigens to polystyrene, and several special adaptations of the a-ELISA. Theory and practice in laboratory indicate that: antigen binding sites must be available in excess to obtain reliable ELISA data; the proportion of antigen that is able to adsorb to polystyrene is independent of input over a defined range of concentration; and above the range of independent binding, adsorption is less reproducible, and the eventual ELISA data more erratic.