Precipitins against influenza A/PR8 were detected in the serum of normal rabbits with a highly sensitive cellulose acetate micro-immunodiffusion technique which allows the diffusion of whole virus particles. Rabbits were susceptible to subclinical artificially induced influenza A/PR8 infection. These natural antibodies might be indicative of natural infection in animal populations with influenza A/PR8 or a closely related influenza A strain. Similar immunodiffusion reactions were observed with normal serum from a sheep, a goat, guinea pigs, and roosters.