Polypeptide fragments obtained by cyanogen bromide (CNBr) cleavage of the hemagglutinin (HA) of the A/Memphis/102/72 (H3N2) strain of influenza virus were inoculated into rabbits, and the resulting antisera were examined for the presence of antibodies capable of reacting with intact virus, detergent-solubilized hemagglutinin, hemagglutinin heavy chain (HA1), or CNBr fragments derived from HA. We have shown that it is possible to raise antibodies to individual fragments including those representing regions of the hemagglutinin molecule that are not normally antigenic within the intact virion. The antibodies are specific for the fragment to which they were raised. In those cases where the immunogen contains carbohydrate, a small proportion of the antibody is directed against these side chains, but this can be removed by adsorption with an unrelated influenza virus. Although only antibodies raised against intact HA are capable of neutralization of virus or lysis of infected cells in the presence of complement, some of the antibodies against fragmented HA possess hemagglutination-inhibition activity.

All of the antibody preparations are capable of binding to purified HA1, but only those raised against fragments containing the antigenic polypeptide, HA1CN1, are capable of binding to intact virus. This indicates that within the intact virion the N-terminal CNBr fragment of HA1 carries immunogenic activity.

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