A Chinese herbal medicine Lu-Shen-Wan, which contains 6 ingredients, is sometimes applied topically to treat boils, carbuncles and other skin infections. 2 elderly women developed a rash after applying Lu-Shen-Wan. Patch tests showed positive reactions to Lu-Shen-Wan and to one of its 6 ingredients, Venenum Bufonis. A study in 15 control patients showed that 13 had positive reactions to 50% Lu-Shen-Wan and 50% Venenum Bufonis but that only 2 showed mild reactions when the concentration of the latter was reduced to 5%. The results suggest that Venenum Bufonis in Lu-Shen-Wan can cause irritant contact dermatitis. The present paper demonstrates that a Western dermatological investigative method, the patch test, is applicable to traditional medicine in confirming the cause of contact dermatitis from a Chinese herbal preparation and in identifying the causative ingredient. Such knowledge is very useful for the improvement of the preparation by reducing the concentration of the irritant, by removing it or by substituting it with an alternative non-irritant substance.