The advent of antiviral chemotherapy provides a strong impetus to develop methods to diagnose viral infections rapidly and accurately. Several other potential contributions of rapid viral diagnoses to patient management also exist. Virus isolation remains the "gold standard" of viral diagnosis against which most newly developed diagnostic approaches--including serologic testing, viral-enzyme detection, microscopic techniques, radioimmunoassays, and enzyme immunoassays--must be compared. Since, as currently performed, enzyme immunoassays such as enzyme-linked immunosorbent assays have reached the limit of their sensitivity, fully satisfactory rapid viral diagnosis will require new approaches. Two such potentially useful approaches are the detection of viral antigen with a method that permits visual localization of virus-specific immunoenzymatic staining and the detection of viral nucleic acids in clinical specimens by hybridization with nucleic-acid probes.