Formalin‐fixed, paraffin‐embedded tissue sections of fifty oral lichen planus lesions including hypertrophic, atrophic and erosive types, were examined by an immunofluorescent technique for the presence of the ABO blood group antigens. The antigen activity in lichen planus lesions was compared with the reactivity of normal human and Rhesus monkey oral mucosa as controls, and the reactivity of adjacent normal epithelium in the same specimen. A difference in the distribution of the blood group substances was observed in the three different types of oral lichen planus. The hypertrophic showed the strongest staining reaction approaching that of the normal, whereas a decrease in quantity of blood group substances was seen in the atrophic, and a tendency to complete loss in the erosive type. There was a marked tendency for loss of staining to be greater in lesions from older patients; this could be related to the greater frequency of atrophic and erosive lesions in the elde