Normal guinea pigs parabiosed for 10 days with guinea pigs tolerant to DNCB-contact sensitivity became completely and permanently unresponsive when sensitized after parabiosis lasting 10 days and partially tolerant when sensitized after parabiosis lasting 2 days. On the other hand, tolerant parabionts became hypersensitive when parabiosed with normal partners which had been sensitized on the day of the operation or before the parabiosis was effected. From these results it is concluded that tolerance to DNCB-contact sensitivity in guinea pigs is a positive phenomenon affecting the afferent branch of the immune response.