Two separate, double-blind studies were carried out in volunteers to compare the reactogenicity of, and serum antibody responses to, whole or ether-Tween-split inactivated influenza virus vaccines. In both studies the ether-Tween-split vaccines induced a lower rate of reactions. The serum hemagglutination-inhibiting (HAl) antibody response of volunteers to the A/Scotland/74 component of the split vaccine used in the first study was significantly greater than that following inoculation of A/Scotland/74 whole-virus vaccine. The neuraminidase-inhibiting (NI) antibody responses of the volunteers to each vaccine were similar. In the second study, a markedly better NI antibody response to the influenza A virus component was seen following immunization with split-virus vaccine, but the HAl antibody response to both split and whole vaccines was the same. In both studies the serum HAl antibody responses to the B/Hong Kong/73 component of the vaccines were similar. Challenge of the volunteers with attenuated influenza viruses homologous to the influenza A component of the vaccines showed both types of vaccines to be protective.