When influenza (H3N2) viruses from infected individuals are grown in embryonated chicken eggs, viruses are isolated which differ antigenically and structurally from viruses grown in mammalian Madin-Darby canine kidney (MDCK) cell culture [G.C. Schild, J.S. Oxford, J.C. de Jong, and R.G. Webster, Nature (London) 303:706-709, 1983]. To determine which of these viruses is most representative of virus replicating in the infected individual, a region of the HA gene of virus present in original clinical samples was amplified by using the polymerase chain reaction and sequenced directly. Comparison of 170 amino acid residues of HA1 flanking and containing the receptor-binding site and antigenic sites indicated that over this region, the HA of virus replicating in the infected individual was identical to that of virus after growth in MDCK cells and was distinct from the HA of viruses grown in eggs. Therefore, cultivation of human influenza H3N2 virus in mammalian MDCK cells results in a virus similar to the predominant population of virus found in the infected individual.