Two parent viruses, A/Finland/4/74(H3N2) and A/Okuda/57(H2N2), virulent and attenuated respectively for man, showed similar differences of virulence in ferrets as judged by estimations of 50% minimal infectious doses (MID50), the level and persistence of nasal infection, the height and duration of pyrexia and the level of lung infection. In ferrets, two recombinant clones, WRL 94(H3N2) and WRL 105(H3N2), were almost as virulent as A/Finland and indistinguishable from one another, a result which agreed well with genetic analysis (Hay et al. 1977); the RNA pieces of these recombinants appeared identical and largely derived from the virulent parent (A/Finland). The results in ferrets did not agree with tests on clone WRL 94 in small numbers of human volunteers but they were not inconsistent with those on clone WRL 105 in larger numbers. It is possible therefore that careful tests in ferrets may yield more accurate information on the virulence of strains than limited tests in human volunteers. A rapid test for virulence in ferrets is described. It could be used to screen many additional recombinants thereby yielding information on the genetical basis of virulence and indicating possible vaccine strains for more thorough testing in ferrets and in man.