SummaryAt regular intervals, new strains of type A influenza virus, immunologically distinct from the previous ones, strike unprotected population.This year, a new strain appeared, the Far-East strain, which rapidly invaded the whole world.After a general survey of influenza, we are presenting our experience with the type A/Singapour/1/57 virus.1. The inoculation of material obtained at the beginning of the disease permitted the isolation of the virus in 12 cases out of 14. Between the 5th and 10th days of illness, the isolation was successful in only 1 case out of 4.Several successive fatal cases were observed. In a case of influenzal encephalitis the virus was isolated from autopsy material. This could indicate the acquisition of neurotropic properties by the influenza virus.2. The evolution of the antibody titers was followed by the complement-fixation and hemagglutination inhibition tests. A significant rise in the antibody levels was generally noted in convalescent individuals. I.H. antibodies attained a mean titer of 320 U/ml between the second and third week from the beginning of the disease. This titer is lowered to 80 U/ml after the tenth week.3. An experimental vaccine was prepared on chicken embryos. The virus was precipitated by the addition of protamine sulfate. The virus-sulfate mixture was washed several times, suspended in a phosphate-buffered solution and finally inactivated by formol. The titer of this vaccine was 320 U.H./ml, and its nitrogen content 50 mg/liter.4. When it was administered to 1,000 individuals, the vaccine caused mild allergic reactions in 5 % of cases. Determination of antibody levels were made 8 or 9 days after vaccination. By mixing different dilutions of serum with different dilutions of virus, it was shown that the complement fixators were parallel to the neutralizing antibodies.These mixtures were inoculated in the allantoid cavity of chick embryos. The serum of vaccinated subjects gave a protection as good as the protection given by convalescent serum.