Two strains of influenza virus A/H3N2 antigenically similar to A/Victoria/3/75 have been isolated in Palermo in February 1977; a peak of extramortality has been observed in the same month, and a rise of HAI antibodies for A/H3N2 viruses in human serum samples, taken from apparently healthy subjects living Palermo, has been demonstrated between October 1976 and February 1977. Similar rise of antibody titers has also taken place in serum samples from two communities in Western Sicily (Favara, 30,000 inhabitants, Favignana, 3,000 inhabitants). Serum samples obtained in June 1977 showed a slight fall of antibody level in Palermo and in Favignana, whereas in Favara a further rise, in comparison to February, is consistent with the hypothesis of a delayed circulation of virus in that community. Reduction of HAI titers for H3N2 viruses was even more pronounced in October 1977 in all groups of serum samples tested. The importance of local surveillance of influenza and the assessment of herd immunity level is discussed.