IgG to internal (NP, M) and external (HA, NA) proteins of influenza virus were isolated from immune rabbit sera using caprylic acid. The IgG retained their specificity and activity, worked in HI, lectin test, and enzyme-immunoassay. IgG migrated towards cathode in electrophoresis on acetate cellulose. Electrophoresis in polyacrylamide gel revealed a heavy chain (55,000) and a light chain (25,000) in IgG molecule. Different methods revealed changes in IgG structure after lyophilization. The possibility of recovering IgG from rat sera using caprylic acid was demonstrated. A comparative analysis of IgG recovered by means of caprylic acid as well as by polyethylene glycol or ammonium sulphate showed the IgG recovered with caprylic acid to contain less admixtures and to be suitable for use in immunological tests without additional purification.