Reply by Dr. Warren

To the Editor.  —It often happens that source material of low potency will induce very mild reactions in mice and rats. In these reactions only one or two joints in the small extremities are affected. The extravasations of red blood cells are minute and difficult to recognize in vivo. The lesions lose color quickly and require frequent observations to detect them. The reaction can be verified histologically by finding capillary damage with extravasation of red blood cells, mononuclear cell infiltration, and synovial injury in the capsular interjoint folds of several contiguous small joints.The reaction induced is enhanced by the more potent surgical specimens, repeated injections, and by rebreeding and interbreeding.I suggest that the reactions can not be sufficiently studied unless a large number of surgical specimens and animals are used and considerable time is given to the experiment.The disease in humans is often confined to a single