Tissue culture techniques are inadequate to diagnose some viral infections. Thus, solid-phase immunoassays have been developed for the direct detection of viral antigens in clinical specimens. While radioimmunoassays (RIA) have attained widespread use, solid-phase enzyme-linked immunosorbent assays (ELISA) offer a number of advantages over RIA systems. ELISAs can be established with approximately the same sensitivity as radioimmunoassays without utilizing unstable, gamma-emitting isotopes. However, before ELISA systems can obtain widespread usage, a number of aspects of the test must be optimized. These include the preparation and use of reagents, the nature of the solid phase, the choice of enzyme, and the enzyme-antibody conjugation method. With the solving of these problems, ELISA should attain widespread usage for rapid diagnosis of a large number of infectious agents.