In genetic recombination experiments with the mouse-lung-adapted human influenza A/Engl/1/61 (H2N2) and an avian influenza strain A/Rostock/34 (FPV) (Hav1N1) which is avirulent for the mouse lung, recombinants in which hemagglutinin and neuraminidase were either segregated (Hav1N2; H2N1) or not segregated (Hav1N1) were selected. The recombinants were studied for mouse-lung virulence and their ability to propagate in mouse kidney cells, mouse embryo fibroblasts, chick embryo kidney cells and chick embryo fibroblasts. An association between plaque formation in mouse kidney cells and mouse-lung virulence was found.