The possibility of practical use of an increase in the level of antibody to a future causative agent of epidemic from minimal to the "critical" level in the human population against the background of a relative epidemic-free state in a town as a prepotent of an epidemic increase of influenza incidence was studied. During 4 spring and 3 autumn seasons (1978--1981) titres of antihemagglutinins to influenza A (H3N2) and B viruses were determined in 200--250 blood serum specimens from residents of Moscow selected with similar age distribution for each season against a working reference human serum. From the analysis of the antibody dynamics influenza A (H3N2) epidemic in 1979 and influenza B epidemic in 1980 were timely prognosed.