An investigation was made of the sensitivity and specificity of four-layer radio-immunoassays (RIA) in the detection of adenovirus, respiratory syncytial virus, influenza virus types A and B, as well as para-influenza virus types 1, 2 and 3 from nasopharyngeal aspirates of 146 patients with acute respiratory disease. The sensitivity of RIA was comparable with that of tissue culture isolation if the total number of positives is considered. The difference may have been caused both by a higher efficiency of the RIA for detection of inactivated or non-cultivable agents and by a higher efficiency of tissue culture methods if the samples contained only small amounts of antigen. Differences between the two antigen detection systems were found in particular with respiratory syncytial virus and influenza B virus. At present, the use of tissue culture isolation together with RIA is the optimal routine laboratory procedure for the diagnosis of respiratory infections.