Serological techniques have been the most widely used means of diagnosing viral diseases for years, but several of their shortcomings have shifted the emphasis toward the direct detection of antigens. Nevertheless, serological testing is still useful if it is impractical or impossible to demonstrate the presence of a viral antigen, e.g., measles virus, togavirus, rubella virus, and Epstein-Barr virus (EBV). Moreover, serological approaches are important in distinguishing between serotypes or isolates. Developments in the design of enzyme immunoassays (EIA) and the production of monoclonal antibodies have permitted the development of techniques that have a specificity approaching or surpassing that required for clinical applications but that do not suffer from the serious drawbacks of classic methods.