Low titers of antibodies reactive with an antigen present in the intercellular (IC) areas and basement-membrane zone (BMZ) of stratified squamous epithelium were observed in the serum of 26% (11 of 42) and 7% (3 of 42) patients with benign prostatic hypertrophy, respectively, by the method of indirect immunofluorescence (IF). IC IF staining could be differentiated from similar IF patterns which are obtainable with sera from individuals with high titers of blood group isohemagglutinins by their stability following absorption with blood group-specific substances A and B. The finding of BMZ antibodies was particularly interesting in view of the suggested association of manifestations of bullous pemphigoid and the occurrence of internal malignancy.