The small but appreciable variations in the physiological potency of commercial d-tubocurarine chloride preparations have been found to be due primarily to the presence of additional quaternary alkaloids which accompany d-tubocurarine through the isolation procedure. Repeated crystallization of d-tubocurarine chloride has yielded a product the essential purity of which has been demonstrated by means of the solubility diagram. The mother liquors have been shown to contain a highly active related quaternary alkaloid which has been named d-chondocurarine. © 1952, American Chemical Society. All rights reserved.