To study the receptor specificity of modern human influenza H1N1 and H3N2 viruses, the analogs of natural receptors, namely sialyloligosaccharides conjugated with high molecular weight (about 1500 kDa) polyacrylamide as biotinylated and label-free probes, have been used. Viruses isolated from clinical specimens were grown in African green monkey kidney (Vero) or Madin-Darby canine kidney (MDCK) cells and chicken embryonated eggs. All Vero-derived viruses had hemagglutinin (HA) sequences indistinguishable from original viruses present in clinical samples, but HAs of three of seven tested MDCK-derived isolates had one or two amino acid substitutions. Despite these host-dependent mutations and differences in the structure of HA molecules of individual strains, all studied Vero- and MDCK-isolated viruses bound to Neu5Ac alpha2-6Galbeta1-4GlcNAc (6'SLN) essentially stronger than to Neu5Acalpha2-6Galbeta1-4Glc (6'SL). Such receptor-binding specificity has been typical for earlier isolated H1N1 human influenza viruses, but there is a new property of H3N2 viruses that has been circulating in the human population during recent years. Propagation of human viruses in chicken embryonated eggs resulted in a selection of variants with amino acid substitutions near the HA receptor-binding site, namely Gln226Arg or Asp225Gly for H1N1 viruses and Leu194Ile and Arg220Ser for H3N2 viruses. These HA mutations disturb the observed strict 6'SLN specificity of recent human influenza viruses.