This is the fourth of a series of studies on the nature of cutaneous idiosyncrasy to arsphenamine, as produced in guinea-pigs. In preliminary experiments,1 made with both intradermal and intravenous injections, it seemed that a locally irritating effect following the second injection was a prerequisite, if a cutaneous flare was to result from a subsequent intravenous injection of neoarsphenamine. In a more recent, unpublished work this conclusion has been found to be false. Carefully controlled experiments on a series of twelve guinea-pigs demonstrated that true sensitivity had developed after but one intradermal injection, since a typical cutaneous flare resulted from a subsequent intravenous injection. The explanation of this variation in results was not immediately apparent. Attention was later called2 to the fact that a diet low in vitamin C apparently increased cutaneous susceptibility to arsphenamine in the guinea-pig, while a diet high in vitamin C seemed to inhibit