The intercellular pattern of fluorescence was demonstrated in 24 of 75 patients with diseases other than pemphigus. A relationship was found between the presence of anti-B isohemagglutinins and intercellular fluorescence as demonstrated by the indirect immunofluorescent (IIF) technique, using fetal rhesus monkey (Macaca mulatta) esophagus as the substrate. When the anti-B isohemagglutinin titer was reduced or abolished, after absorption with B substance, the intercellular pattern was reduced considerably or abolished. The importance of the blood type of the substrate employed in the IIF technique and the blood type of the patient is stressed so that these factors which may give rise to error in the interpretation of intercellular fluorescence are excluded.