Groups of student volunteers were immunized with one of five different inactivated influenza virus vaccines. The concentration of virus in the various vaccines differed by both the international unitage test and by the concentration of haemagglutinin, as measured by the single radial diffusion test; the results of the two methods of standardization showed no correlation. The serum HI response to immunization was variable; volunteers given A/England/72 showed a 16.6-fold increase in homologous serum antibody titre whilst volunteers given A/Hong Kong/68 vaccine showed a 4.2-fold increase. The variable response of volunteers to immunization could not be explained by the varied concentration of virus in the vaccines, as measured by either test, the titres of serum HI antibody present before immunization, or a combination of these two factors.The ability to infect volunteers with WRL 105 virus 4 weeks after immunization with heterologous, inactivated virus vaccine was directly related to the degree of cross-reactivity between the haemagglutinins of this vaccine virus and WRL 105 virus. Thus, the greatest number of infections by the challenge virus were seen in volunteers given A/Hong Kong/68 vaccine, less were observed in volunteers given A/England/72 vaccine, and least were found in groups given A/Port Chalmers/73 or A/Scotland/74 vaccine. However, compared with the incidence of infection in volunteers given B/Hong Kong/73 vaccine, all the heterologous influenza A vaccine gave some immunity to challenge infection.