Hairs of the Thaumetopoea pityocampa caterpillar (Lepidoptera) cause a cutaneous reaction in man and animals. The irritating fraction extracted from hairs contains soluble proteins which were separated by various electrophoretic and immunoelectrophoretic techniques. Some of these proteins are present also in cuticle and haemolymph. One protein of 28,000 mol. wt is hair specific and caused a reaction in pig skin identical to that produced by hair extract. It is therefore an urticating protein which we have named thaumetopoein. This protein is formed of two subunits of molecular weights 13,000 and 15,000. It is present in large quantities in the glands producing urticating hairs.