To study the effect of indomethacin treatment on the reactivity of contact hypersensitivity and discuss relevant mediators which could affect contact hypersensitivity, the following items were evaluated: the change in the plasma concentration of prostsglandin E following indomethacin treatment; the change of contact hypersensitivity following indomethacin treatment. The results ore summarized as follows: 1. The plasma level of prostaglandin E in indomethacin treated mice decreased. 2. The contact hypersensitivity of indomethacin treated mice was depressed. Considering the suppression of contact hyper sensitivity by indomethacin treatment, the metabolites of cyclo-oxygenase such as prostaglandin E may be the possible mediators of induction of contact hypersensitivity. (Kor J Dermatol 28(3): 278 282, 1990)