Microplate enzyme-linked immunosorbent assays (ELISA) for viral diseases were investigated with special reference to rubella. Standardization of the carrier plates, antigens, conjugates, and substrate was found to be essential. The ELISA results were compared with results of hemagglutination-inhibition tests for rubella, and ELISA was used in an epidemiologic study. Antibodies to cytomegalovirus, measles, adenovirus, coxsackieviruses, and herpesviruses were also assayed by ELISA. ELISA has veterinary applications in detection of respiratory syncytial virus and Newcastle disease virus, and it has been used for assay of plant viruses.