Monoclonal antibodies were used to compare the antigenicities of the haemagglutinins of two receptor binding mutants of X-31 (H3N2) influenza virus. The mutants which differed from each other in recognizing sialic acid in either alpha 2-6 linkage or alpha 2-3 linkage to galactose also differed exclusively at residue 226 of the HA1 polypeptides of their haemagglutinins (G. N. Rogers, J. C. Paulson, R. S. Daniels, J. J. Skehel, I. A. Wilson, and D. C. Wiley, Nature (London) 304, 76-78, 1983). The results obtained indicate that whereas the majority of antihaemagglutinin monoclonal antibodies react with both mutants equally well, a number which specifically recognize residues 193, 199, 219, and 229 of HA1 of the alpha 2-6 linkage-binding mutant fail to recognize the alpha 2-3 linkage-binding haemagglutinins. The results are discussed with reference to the three-dimensional structure of the haemagglutinin and in relation to differences in antigenicity observed in the haemagglutinins of viruses grown in cells of different types.