The advantages of enzyme-immunoassay (EIA) over radioactive assay techniques are mainly convenience in use, in that the labelled immunoreagents are stable for long periods, and the precautions and disposal procedures required for radioisotopes are unnecessary. In addition, the use of chromogenic substrates for the enzyme labels permits visual interpretation of test results in some cases. The only real disadvantages of EIA tests are the loss of antibody reactivity that may result from conjugation to enzymes, and the limits of substrate detection. For example, use of enzymes that have molecular weights higher than that of IgG molecules such as B-D-galactosidase (mw 540,000 daltons) can cause steric hindrance of antibody activity. With regard to limits of substrate detection, improvement of enzyme detection by use of fluorogenic, luminescent, or radioactive substrates has been proposed. The major emphasis of the report will be on current developments in EIA methodology and the application of EIA to diagnosis of infectious diseases.