Vesicular and bullous dermatitis among celery harvesters became a subject of special interest to us in early 1959. The stimulus was occasioned by an inquiry from a Michigan practitioner1 who wanted to know why the celery harvesters in his community were developing severe vesicular and bullous lesions, and whether or not contact with disouteased celery could be the cause of the eruption. Not having investigated any outbreaks of occupational dermatitis from celery, we reviewed the medical literature available to us. At the time, all of the references consulted emphasized the cutaneous sensitizing capacity of celery oil. Shortly thereafter, Dr. H. A. Tucker,2 well-known for his interest in psoralens and radiant energy, suggested that photosensitivity to furocoumarins or psoralens contained in celery might account for the dermatitis observed in Michigan. Continued search of the literature provided ample evidence that the following theories could be advanced concerning the cause of