Both recombinant viruses grew to equivalent high titres in MDCK cells and haemagglutinated chicken red blood cells with a geometric mean titre (GMT) of 128. Following incubation of the microtitre plates at 378C, wells containing high titres of virus showed haemagglutination reversal. This is accounted for by the digestion of the sialic acid receptor from the erythrocyte surface by active NA enzyme. The ability of the wild-type pH1N1 to elute virus from red blood cells was 4-fold greater than that of the H275Y isogenic mutant ( Table 1) .