Glyceryl monothioglycolate has recently been implicated as an allergen. A case of a hairdresser who used this material is presented. He appeared to have an irritant contact dermatitis on his fingertips. Patch testing the patient and extensive patch testing in a group of fifty-four volunteers showed a very high incidence of irritant positive patch tests. No instance of allergy could be demonstrated in the reading of the patch tests or in biopsies of several individuals. Differentiation between allergens and irritants is difficult at times, but is necessary to avoid mislabeling of chemicals.