P-carbaminophenylarsonic acid (H2O3As.C6H4.NHC ONH2) is a comparatively recent addition to the armamentarium of the modern medical practitioner. Its use dates back to January 1932, when it was introduced by Reed, Anderson, David and Leake,1although experimental studies were started at least two years previously. This compound is marketed by Eli Lilly & Co. under the name of carbarsone and is supplied in capsules of 0.25 Gm. (3¾ grains) each. It contains 28.85 per cent arsenic and has been accepted by the Council on Pharmacy and Chemistry of the American Medical Association.2 Although this product is an arsenical closely related chemically to tryparsamide and acetarsone ("stovarsol"), it is surprisingly nontoxic. To date, no serious reactions or deaths have been reported following the rectal or oral administration of this drug. In general, the pentavalent arsenicals, including tryparsamide and carbarsone, are