Abstract: A method for destroying microbial contamination, such as viral and bacterial contamination and mycoplasma contamination, in protein material, and particularly tissue and serum from animals and human beings. The method comprises the reducing of the temperature of the protein material, in a state other than a lyophilized state, to at least the freezing point and preferably to the eutectic point. The method comprises reducing the temperature of lyophilized protein to at least a temperature of -3 degrees C., or colder. Thereafter, gamma radiation is applied in an amount sufficient, at least 5,000 rads and preferably at least 600,000 rads, to destroy substantially all microbial contamination in the protein material without significantly reducing the protein efficacy.