A potent formalin-killed vaccine was prepared from bovine kidney cultures infected with the SF-4 strain of Myxovirus parainfluenza 3 virus. The aqueous vaccine was highly potent for guinea pigs when administered by various routes and regularly evoked the formation of neutralizing, HI and CF antibodies. On the other hand, the aqueous vaccine was of insufficient potency to immunize cattle in the dosage employed. Emulsification of the vaccine with Arlacel A-mineral oil adjuvant effected great enhancement of antigenic potency. Such material, in properdosage, evoked high level antibody response in both guinea pigs and bovines. The significance of the findings is discussed.

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