A monoclonal antibody raised against X-31 influenza virus reacted with the majority of natural H3N2 viruses isolated between 1968 and 1982. A number of variants of X-31 and of a receptor-binding mutant of X-31 were selected by the antibody during virus replication in eggs and MDCK cells. Antibody-binding assays indicated that the viruses selected were not antigenic variants and analyses using derivatized erythrocytes showed that their receptor-binding properties differed from those of the parent viruses. The amino acid substitutions in the variants were all located in the vicinity of the receptor-binding site and the structural consequences are discussed in relation to the three-dimensional structure of X-31 HA. In addition all of the variants fused membranes at higher pH than wild-type virus indicating that structural modifications in the distal globular region of HA influence the low pH-induced conformational change required for membrane fusion.