The regions of antigenic variation in influenza hemagglutinin (HA) are located on surface-accessible regions in the three-dimensional structure of the HA1 monomer. The aim of this study was to establish whether a novel variant virus, IMUT4, in which we had mutated specific amino acid residues (HA1 63, 144, 158, and 193) in these regions, previously shown to be immunodominant for CBA/Ca mice, would either (a) establish holes in the antibody (ab) repertoire or (b) preclude further antigenic variation in IMUT4. CBA/Ca mice were able to mount a neutralizing ab response to IMUT4 infection and molecular recognition sites were established by sequencing of the HA genes of monoclonal antibody (mAb)-selected laboratory variants of wild-type X31 virus (HA1 131, 145, 155, and 196). However, each of these mAbs failed to select further antigenic variants of IMUT4, in ovo, but rather a receptor binding mutant (HA1 190 Glu-->Asp; 226 Leu-->Gln) that was still recognized by the selecting mAb, specific for HA1 155 of X31 virus. The facility for antigenic variation in influenza would appear to be compromised, therefore, by targeted mutation of immunodominant sites, as initially proposed by S. Fazekas de St. Groth (Fazekas de St. Groth, S. 1977. Antigenic, adaptive and adsorptive variants of the influenza haemagglutinin. In Topics in Infectious Diseases. Vol. 3. R.G. Laver, H. Bachmayer, and R. Weil, editors. Springer-Verlag, Vienna. 25-48.). It is interesting to note that recent isolates of the H3 subtype, (e.g., A/Beijing/92) obtained between 1991 and 1993, contain the same substitutions at HA1 190 and 226, which may indicate similar constraints to immune evasion and the relevance of our findings to antigenic variation in the human population.