Summary Immunologic adjuvant 65 caused a strong enhancement of antibody response to influenza virus antigens in tests in animals. Complexed synthetic polynucleotides Poly I:C (rI:rC) and Poly A:U (rA:rU) produced comparatively weak potentiation of antibody response to these antigens and in certain instances, Poly A:U was inhibitory. Immunologic adjuvant 65 combined with the Poly I:C or Poly A:U acted synergistically to cause a hyperpotentiation of antibody response which greatly exceeded the additive effects of adjuvant and complexed polynucleotides tested singly. Such potentiation did not occur when the complexed polynucleotides were tested with alum adjuvant or when Poly I or Poly C was added alone. Poly I:C was generally more effective than Poly A:U. Maximal potentiation was demonstrated at 26 μg/ml of Poly I:C and there was no further enhancement when the concentration was raised to 260 μg/ml. Use of adjuvant 65 combined with complexed polynucleotides promises to contribute a new magnitude of immunizing capability to be obtained when used with influenza and other killed antigen preparations.