The effectiveness of vaccines against Asian strains of influenza virus has been studied by four groups of investigators in a comparison of 12,002 test subjects who received certain monovalent or polyvalent vaccines with 9,363 contratest subjects who received placebo injections. The estimates of effectiveness, based on relative respiratory illness rates, ranged from 42 to 67% in the various groups. The trends are sufficiently distinct to indicate the final effects which may even be increased when laboratory studies identify and eliminate cases of respiratory disease caused by agents other than the Asian virus. They also indicate that doses of 200 CCA units given subcutaneously were less than optimal, and that vaccines at 400 CCA units or above gave higher and more uniform antibody levels. It is recommended that an Asian virus vaccine of 400 CCA units be used for primary immunization.