Therapy-resistant plantar warts of 8 patients have been topically treated with diphencyprone, a new potent contact allergen. Three of the patients showed only one single aggregation of warts, which completely disappeared within two or three months after start of treatment. In the other five patients who had multilocular warts, a controlled trial was performed. In three of these patients only the treated warts resolved whereas the untreated ones persisted. In one case all warts disappeared already during the period of sensitization, and in the fifth patient the warts persisted without any change.