In the present work, the contact hypersensitivity skin test reaction to picryl chloride in CBA mice was examined. The test was performed by applying the contact allergen to the ear skin and making a series of histological analyses up to 24 hours after challenge. An increment in ear thickness, measured with an engineer's micrometer 24 hours after challenge, was obvious in a group of sensitized mice when compared with a nonsensitized control group, and the difference was found to be highly significant. One hour after challenge, mononuclear cells appeared in the dermis, increasing in numbers during the following 12 hours. At this time, neutrophil granulocytes were the dominant cells in the infiltrate and remained so up to 24 hours after challenge. On the basis of the experiments performed here we conclude that measuring of the ear swelling with a micrometer 24 hours after challenge is a useful and reliable test of contact sensitivity in the mouse.