Neutralization-resistant variants of influenza C/Ann Arbor/1/50 virus were selected with monoclonal antibodies against four different antigenic sites on the hemagglutinin-esterase (HE) glycoprotein, and their HE genes were sequenced to identify amino acid residues important for the integrity of each site. Twelve different amino acid substitutions in a total of 18 antigenic variants were all located on the HE1 subunit. Although variants for antigenic site A-2 had a change at position 367, all substitutions in the variants for sites A-1, A-3, and A-4 occurred in the central region of the HE1 spanning amino acid positions 178 to 283. Furthermore, it was found that many of the substitutions in the variants selected with antibodies to sites A-1 and A-3 were clustered within or near one of the three variable regions revealed previously by comparing amino acid sequences of the HEs among various influenza C isolates (Buonagurio, D. A., Nakada, S., Fitch, W. M., and Palese, P., Virology 146, 221-232, 1985). The antigenic variants were also examined for their ability to agglutinate chicken and human erythrocytes in order to obtain information concerning the receptor-binding site on the HE molecule. The results suggested that the amino acid changes at residues 178, 186, 187, 190, 206, 212, and 226 decreased the hemagglutinating activity whereas those at residues 245, 266, and 283 produced an opposite effect.