Malarial relapse after a standard course of quinine occurs in a proportion of all cases treated. While this may be due to variations in the virulence of malaria parasites, or variations in the susceptibility of different individuals, it may also be due to variations in the concentration of quinine in the blood of patients rcceiving the same therapeutic dose. A simple and accurate method of determining the amount of quinine in the blood might therefore serve as a guide to treatment. Several investigators have previously described methods for the determination of quinine in the blood, none of which have been applicable to clinical observations either because they have been too time-consuming or because they did not recover all of the quinine. Ether is the solvent that has been most generally used in extracting the alkaloid from the blood, after the addition of concentrated ammonia.