The National Institute for Medical Research, London, was asked by the WHO Expert Committee on Biological Standardization to take steps to set up an international standard of streptokinase-streptodornase. This paper describes the nature and handling of the material and its establishment as the International Standard for Streptokinase-Streptodornase, with a defined potency of 3100 International Units of Streptokinase and 2400 International Units of Streptodornase per ampoule. The International Unit of Streptokinase is defined as the activity contained in 0.002090 mg of the International Standard for Streptokinase-Streptodornase. The International Unit of Streptodornase is defined as the activity contained in 0.002700 mg of the International Standard for Streptokinase-Streptodornase.