In the winters of 1975-76 and 1976-77 there were fairly heavy epidemics of influenza in Japan, and on those occasions considerable discussions were held on the efficacy of influenza vaccine in connection with the variability of antigenic structure of the influenza virus. The type of the virus which prevailed in the winter of 1975-76 belonged to type A, but detailed antigenic analysis revealed that there had been two main antigenic types somewhat but definitely distinguishable from each other, the one is the well-known A/Victoria/75 type while the other is A/Tokyo/75 type. This epidemic seems to have started with the latter type, but soon the two types of virus prevailed hand in hand until the late part of the epidemic when the type A/Victoria/75 became overwhelming. The virus for the vaccine was however A/Tokyo/6/73 for that season, which is antigenically considerably different from both of these two strains. In the beginning of 1976, namely at the end of the epidemic season of type A, a few scattered epidemics due to type B influenza were reported from some parts of Japan. The result of antigenic analysis found that the virus belonged to B/Hong Kong/72 type, though showing some degree of antigenic schift. The same type of virus caused the epidemic of the winter 1976-77, which was considerably wide-spread and developed unexpectedly large number of cases. There were active discussion on the effectiveness of vaccine, of which B type was B/Gifu/1/73, recovered from the fairly large epidemic in 1973, and belonging to B/Hong Kong/72.