Summary Ninety-six strains of influenza A and 113 influenza B were found in the two MRC/PHLS surveys. In the general practitioner survey the majority of strains were isolated from cases diagnosed as influenza but in hospital most came from cases diagnosed as having other respiratory infections. In the latter survey isolation rates were higher in upper respiratory tract infection than in lower ones but in the general practice survey this trend was reversed. Influenza-like symptoms tended to increase with age, while coryza and vomiting was commoner in the younger age groups. The frequency of influenza virus infections found in the survey is compared with those found in Glasgow over the last decade and their effect on the incidence of lower respiratory tract infection discussed.