Experimental inactivated influenza vaccines prepared with strain X 53, derived from A/New Jersey/76 virus were injected to several groups of subjects. Different compositions of vaccine were used and the subjects were selected from different age groups. Antibody responses were measured 15 days after vaccination; it was then possible to evaluate the conversion rates, the percentages of subjects showing a protective level of antibodies and the average level of antibodies. The results show that a very small dose of antigen, probably acting as a booster, gives very good results in the older age group. On the other hand, young adults (below 23) react to a much lower degree to the vaccine. The results are discussed in relation to the age distribution of preexisting antibodies in the experimental groups and in the general population in France.