Guinea pigs sensitized to dinitrochlorobenzene can be more or less completely desensitized by 60 daily applications of 1% solution of dinitrochlorobenzene in acetone to one nipple. De-sensitization was proved by (1) patch tests on the flank and (2) by histologic examination of the untreated nipple after the eczematogen had been applied once. An attempt at desensitizing a man who was hypersensitive to dinitrochlorobenzene has been tried without success.