The diallylglycol carbonate monomer causes dermatitis due to irritation in the optical industry. Cutaneous intolerance may effect as many as 70% of the exposed persons employed. Almost all control subjects who where patch-tested showed irritation at a 2% concentration. The histological effects were an acute oedema with inflammation of the papillary dermis, and diapedesis of neutrophil polymorphonuclear leukocytes. Experiments on animals confirmed the irritant nature of the product; in the rabbit, a single application produced irritation, but to a lesser degree than in humans. Tests for possible sensitizing effects in the guinea pig have given negative results. Chemical analysis of the monomer revealed the presence of diallyl carbonate and acrolein. Allyl alcohol was detected in only one case. Patch tests were carried out in a group of control subjects with acrolein, diallyl carbonate and allyl alcohol. The histological appearance of the lesions caused by acrolein was quite different from that due to diallyglycol carbonate. It is probable that the irritant is the diallylglycol carbonate monomer itself.