This paper deals with immunologic prophylaxis of Rhus dermatitis due to Rhus radicans (poison ivy) and Rhus toxicodendron (poison oak). A comprehensive summary of the clinical biology of Rhus dermatitis has been given in a previous paper, which forms the prelude to the present theme.1 A striking feature of the literature on Rhus prophylaxis is the contradictory nature of many reports. Students of the subject hold diametrically opposed views. Some achieve remarkable results which are just as remarkably denied by others. Despite genuine advances in the basic knowledge of allergic contact dermatitis, the issue of hyposensitization is one in which there is no end of clinical testimony but relatively little in the way of objective research. Therapeutic ambitiousness has colored the picture with extravagant claims. The paramount aim of the present effort is to test the soundness of the concept of hyposensitization and