Diagnostic attempts by indirect immunofluorescence (IF) were made for parainfluenza‐, respiratory syncytial (RS)‐ and adenovirus on laryngo‐pharyngeal cells aspirated within a few hours of admission from 87 patients with a diagnosis of croup. Parainfluenza virus antigen was demonstrated in samples of secretion cells from 33 out of 44 patients from whom parainfluenza virus was isolates in tissue culture. Furthermore, in six patients parainfluenza virus infection was demonstrated serologically. RS‐virus was never isolated. One diagnosis of RS‐virus infection made by IF was confirmed by the complement fixation (CF) test. Adenovirus was isolated in eight cases, none of which were diagnosed by IF, in spite of one showing titre increase in the CF‐test. Paired sera were available from 52 of the 87 patients. No influenza orMycoplasma pneumoniaeinfections could be diagnosed by the CF‐test. The IF‐technique appears to be an important supplement to the rapid diagnosis of virus infections in patients suffe