It has previously been shown tha UV irradiation can give rise to fresh skin lesions in the uninvolved skin of patients with pemphigus. In this study no increase of acantholytic activity was found in an organ culture model system for pemphigus in vitro, after in vitro and in vivo irradiation under various experimental conditions. By immunofluorescence no increase in IgG binding in irradiated specimens was observed. From these in vitro findings, which contrast the effect of UV light in pemphigus in vivo, it may be concluded that the circulatory system of the skin plays a major role in the photo induction of acantholysis in pemphigus.