Monoclonal antibody (MoAb) IIF4 defines an epitope on the HA2 part of influenza haemagglutinin (HA) (Russ et al., 1987). It was also found this epitope becomes fully accessible after pH 5 treatment of the antigen and is shared by strains of H3 subtype. In this study we found binding of MoAb IIF4 also to some strains belonging to H2, H4, H7, and H10 subtypes. We prepared rabbit polyclonal anti-IIF4 anti-idiotype (anti-Id) antibody. In competitive assays, the inhibition potential of anti-Id was considerably higher than that of native HA. Anti-Id was used for the preparation of mouse Ab3 (anti-anti-IIF4) serum. Reactivity pattern of Ab3 with influenza virus strains differed from Ab1 in (i) appearance of binding to some strains of H2 and H7 subtype and (ii) decreased dependency of Ab3 binding on the pH forms of antigen. The reactivity of Ab1 and Ab3 with two amantadine-resistant virus mutants indicates that IIF4 epitope (and its related region recognized by Ab3) becomes accessible in consequence of destabilization of trimeric arrangement of HA and it also correlates with expulsion of N-terminus of HA2.