Details are given of a number of experiments, carried out in 1957 in Japan among high school students, factory workers and student nurses and in military camps, to test the efficacy of Asian influenza vaccine at various strengths and doses and prepared from different virus strains. The Asian influenza virus strains used were A/Adachi/2/57, A/Kumamoto/Y5/57 and A/Kumamoto/K9/57. Results were tested by the haemagglutination-inhibition reaction.Antibody response to Adachi-strain vaccine was very satisfactory, particularly when inoculated at a strength of 300 CCA units per ml in two doses of 0.5 ml each. Monovalent Adachi-strain vaccine gave better results than a trivalent vaccine containing Adachi strain, an earlier A strain and a B strain in equal amounts. Vaccine prepared from the Y5 strain, considered representative of extreme Q-phase virus, was less effective than Adachi vaccine.