Serotherapy still remains a way of treatment in some diseases, and it could be consider superior to any other mode of action because the protecting substances of the body are the products of the organism itself. The aim of the study was to establish an "in vivo" method for testing the efficacy of therapeutic serum. Hyperimmune serum for influenza A/PR8/34 viral strain, was prepared in sheep, and tested for inhibition of haemagglutination and microneutralisation. Seroprotection was evaluated in mice one day after being challenged with a lethal dose of the same virus. Our study shows that protection occurred in all mice treated with undiluted hyperimmune serum one day post infection (no clinical signs, faster recovery of the body weight after the first three days of the infection, all mice survived).