Over the past 10 years the diagnosis of viral pneumonias and other infections of the respiratory tract has been greatly facilitated by the application of new biotechnology. Molecular and immunologic techniques have been developed for the detection of viral nucleic acids and proteins in clinical materials obtained from the lung, either by bronchoalveolar lavage or biopsy. Clinicians are now able to obtain a virologic diagnosis with a high degree of sensitivity and specificity, often within a few hours of the diagnostic procedure. We review the current status of the newer molecular and immunologic modalities being applied to the rapid laboratory diagnosis of viral infections of the lung, including direct and indirect fluorescent antibody staining of material from the respiratory tract, enzyme immunoassays, centrifugation cultures, and molecular techniques, such as the polymerase chain reaction. These techniques permit the rational use of specific antiviral therapeutic agents in patients with drug-sensitive pulmonary viral infections, thus improving both morbidity and mortality.