Summary A new immunologic adjuvant, adjuvant 65, is described and the results of extensive tests of polyvalent influenza virus vaccine in the adjuvant are presented. The vaccine consists of a water-in-oil emulsion in which the adjuvant portion consists of peanut oil, mannide monooleate and aluminum monostearate. The adjuvant effect measured in terms of hemagglutination-inhibition antibody responses 28 days after vaccination in mice or guinea pigs is roughly equal to that obtained with Freund's incomplete mineral oil adjuvant. Adjuvant 65 influenza vaccine showed striking enhancement and long-term persistence of antibody in guinea pigs, rabbits and monkeys compared with the corresponding aqueous vaccines. The findings in histomorphologic-pathologic studies in animals and the clinical and serologic results obtained in large-scale studies in man are presented elsewhere. Adjuvant 65 shows promise of meeting the present urgent need for a safe and effective adjuvant for enhancing immunologic responses to viral and other vaccines and possibly also for treating allergic disease.