Two years ago I1 presented the first of a series of papers on the microchemical study of arsenic in the tissue. The method of preparation was described in detail, and numerous controls and proofs of arsenic were given. Since that time a large number of sections have been examined with adequate controls, and the method has been eminently satisfactory. Two slight changes have been made in the method as originally used. Instead of incubating the pieces of tissue at 70 to80 C. for four days, it was found that incubation at 56 C. was sufficient if the tissue was cut into small pieces. Considerable staining power was retained. Hemalum, instead of hematoxylin, was used as stain, with better results. In the previous paper,1 the results of a study of arsenical keratoses and pigmentation were given. Since then, numerous sections of the same conditions have been examined with practically identical results. Nearly all of the arsenic was localized in the epidermis, subpapillary layer and other ectodermal tissues, such as the sweat and sebaceous glands and their ducts, and the hair follicles. In every instance the drug administered contained quintavalent arsenic, such as solution of potassium arsenite (Fowler's solution) and solution of arsenous and mercuric iodide (Donovan's solution), arsenic acid, sodium and iron cacodylate. None of the patients had received trivalent arsenicals such as the various arsphenamines. Numerous writers