This small book on bio-assay is divided into three parts. The first part discusses general principles. The second part takes up the substances now recommended for systematic biologic standardization by the Committee of Hygiene of the League of Nations, which aims at the development of international units. It deals with the digitalis series, epinephrine, hypophysis, ergot, arsphenamines, insulin, thyroid and anthelmintics. The third part is devoted to the biologic standardization of substances not yet considered by the international commission: antipyretics, miotics and mydriatics, vasoconstrictors and vasodilators, local anesthetics, and a mixture of the opium alkaloids.