The development of procedures for the use of the K-II continuous flow zonal ultracentrifuge has resulted, for the first time, in the availability of large quantities of highly purified influenza virus vaccine essentially free of nonviral egg-derived material. With these vaccines, it was determined that the inherent toxicity of purified virus strains and combinations thereof is uniformly low, and that the variable side effects frequently seen with conventionally produced vaccines are not virus related. Purified vaccines are equal in antigenicity to equivalent doses of conventional vaccines, demonstrating that nonviral components of conventional vaccine have no beneficial effects on vaccine antigenicity. Purified vaccine of twice normal potency is no more reactogenic than the same vaccine in standard doses, indicating that future vaccines of even higher potency and low reactivity are well within the realm of possibility.