A monoclonal antibody, LMBH6, was derived from mice which had been sequentially immunized with bromelain-cleaved haemagglutinin (BHA) from influenza virus A/Aichi/2/68, A/Victoria/3/75 and A/Philippines/2/82 (all H3N2). LMBH6 recognizes the haemagglutinin (HA) of all H3N2 influenza A strains tested, which were isolated between 1968 and 1989. HA in the low-pH-induced conformation is not recognized, and cleavage of the HA0 precursor to HA1 and HA2 is needed to obtain efficient binding. Compared to other monoclonal antibodies, binding of LMBH6 to virus and to virus-infected cells is weak, while binding to BHA is comparable. Electron microscopy demonstrates binding to the membrane proximal end of the stem structure. The antibody shows no haemagglutination-inhibition activity, but inhibits polykaryon formation and the low-pH-induced conformational change of BHA. However, LMBH6 cannot prevent infection of MDCK cells but slows the growth of virus when included in a plaque assay overlay.