In our first paper in this series on narcotic drug addiction,1we dealt with the question whether the continuous taking of morphin causes the presence in the blood serum of any substance which has a protective effect against morphin, and arrived at negative conclusions. Another question is whether any specific toxic substance is produced by morphin habituation. This question, which is closely related to the first, has a more direct practical bearing on the current theories of the nature of morphin addiction and the causation of withdrawal symptoms, and also on treatment. There has been considerable controversy on this subject, and a more or less widespread belief that there is such a substance and that it is the cause of the well known symptoms of morphin withdrawal when no longer neutralized by the continued administration of morphin; and some methods of treatment of drug addiction have been founded upon this