Patients with contact photodermatitis from bithionol fall into two groups: (a) Transient contact reactors with normal MED.* (b) "Persistent light reactors" with lowered MED and positive WGT. Patch tests to bithionol are usually negative but photopatch tests to bithionol are positive and eczematous. Photosensitivity occurs not only from exposure to the usual sunburn spectrum, but also to longer wavelengths of light as demonstrated by reactions through window glass. Red veterinary petrolatum is an effective sunscreen, preventing both the ordinary sunburn and burning from wavelengths of light above 3,200 angstroms. Parenteral use of psoralens and subsequent sunlight exposure can convert the lowered MED to normal range.