The new methods of rapid viral diagnosis make it possible to specify a number of the most prevalent agents of respiratory tract infections within 24 hours. The techniques are based on the immunological detection of antigens of respiratory syncytial (RSV), adeno, parainfluenza type 1, 2 and 3, as well as of influenza A and B viruses in nasopharyngeal secretions. During a one-year period we have used these methods to evaluate diagnostically 1541 outpatients presenting with upper and lower respiratory tract infections. The patients included babies, infants and children under 16. In about 50% of all sick babies below the age of three months a definite viral infection could be established, and in approximately 30% of infants and children aged up to 4 years. RSV was most frequently observed, accounting for 53.6% of all infections (80% of all babies below the age of 3 months, in whom specified agents could be identified, had RSV infection). The next most frequent pathogens were parainfluenza type 3 (18,8%), influenza A (11,3%) and, finally, adenoviruses (10.2%). The epidemiological and clinical characteristics of these infections are summarized. In addition, the results of these antigen detecting assays have been compared with those of concomitantly conducted virus isolation techniques in cell cultures. This comparative analysis most impressively revealed the time saved by attempting an etiological diagnosis using the antigen detecting system: in only 6% was a specific diagnosis established on the basis of virus isolation, whereas the delay was equal or more than 8 days in 36% of all patients enroled.(ABSTRACT TRUNCATED AT 250 WORDS)