Virus titers of recombinant influenza viruses. MDCK cells were infected with rgH1N1, rgH3N2, or rgH1N1(H3N2-PB1) at m.o.i. = 1, and the supernatant was harvested at 9 h p.i. The mean virus titers are given in percent as well as plaque forming units/ml. The error bars were derived from three independent experiments.