Distribution of h.i.a. against A/New Jersey/8/76 (Hsw1N1), A/PR/8/34 (H0N1), A/FM/1/47 (H1N1), A/Japan/305/57 (H2N2) influenza virus strains was determined in 553 serum samples, previously tested for A/Hong Kong/1/68 virus, collected in the late summer of 1977 in Milan and Bari. For all variants, distinctive antibody patterns were obtained which reflect periods of prevalence of these viruses in man. So, the peak of positivity and the higher h.i.a. levels were found for A/New Jersey/8/76 strain in subjects born between 1907 and 1916, in the case of A/PR/8/34 virus for the birth years 1927-36, ten years later (1937-46) for A/FM/1/47 strain and for A/Japan/305/57 and A/Hong Kong/1/68 variants respectively for the birth years 1947-56 and 1962-66. The presence of antibodies to influenza A viruses not supposed to have been current during the person's lifetime has been discussed.