Genetic recombination of low egg-passage, low-yielding, heat-inactivated HK/Aichi/68 influenza virus and a standard, high-yielding laboratory-adapted strain (A0/PR8/34) resulted in the isolation of a recombinant (X-31) antigenically indistinguishable from the Aichi virus but with hemagglutinin-yielding capacity. in the chick embryo comparable to that of PR8 (i.e., 16- to 32-fold greater than Aichi). Other nonantigenic markers inherited by X-31 from PR8 include spherical morphology and virulence for the lungs of mice. A pilot commercial production lot of X-31 inactivated vaccine was administered to 16 human subjects of various ages; 87% responded significantly as measured by at least one of nine different serologic tests. These results demonstrate the feasibility of using genetic recombination for the deliberate design of vaccine viruses with the desired optimal characteristics.