The effect of an allergic contact dermatitis on the irritant was studied using DNCB(2. 5% in acetone) as the sensitizing agent and sodium lauryl sulfate of various concentrations (0. 5, 1. 0, 2. 0% in Vaseline) as the irritant. Three groups of 30 albino Hartley strain female guinea pigs weighing between 350 g and 500 g were used. The results were as follows: 1. The animals which were sensitized with DNCB were shown to be significantly(p<0.05) more irritable than control animals. 2. The animals which were sensitized and challenged with DNCB were shown to be significantly(p<0. 05) more irritable than control and sensitized animals. These data suggest that allergic contact sensitivity as well as allergic contact dermatitis lower the threshold to a nonspecific irritant in guinea pigs, which may explain a mechanism of 'angry back syndrome.