There has been increasing interest in characterizing the sensitizing moiety of thimerosal [TIM]. following the finding that patients with photosensitivity to piroxicam arc allergic to the thiosalicylic acid [TIO] moiety of TIM. For this purpose, the authors have conducted 2 studies in TIM-sensitive patients. In the 1st. of 175 patients tested with TIO and ammoniated mercuric chloride [HGAM|, 45.7% reacted only to TIM, whereas 45. 7%. reacted also to TIO and 17.7%. also to HGAM: 9.1% reacted to both TIO and HGAM. In the 2nd. of 47 patients tested with TIO and ethylmercuric chloride [F.THG], 87.2% reacted to ETHG. 44.7% to TIO and 31.9% reacted to both. None of the patients reacted only to TIM. The authors conclude that thimerosal allergy is due either to the mercuric moiety or to thiosalicylic acid, with no cases of sensitivity only to the whole molecule of TIM. TIM-sensitive patients are mainly allergic to the mercuric moiety. but among them there are a large number of TIO-sensitive patients, and these should be advised to avoid piroxicam.