The cytoplasmic sialomucin in Paget cells of extramammary Paget's disease was examined by means of a battery of histochemical techniques. The staining methods used involved an electrolyte--Alcian Blue (pH 5.8), periodic acid--Schiff and azure A at selected pH levels. Methylation, saponification, borohydride reduction, acid hydrolysis, and digestion with diastase, neuraminidase (Vibrio cholerae) or chondroitinase ABC, were also employed. The cytoplasmic mucin was found to exhibit positive reaction for the above staining which were variously altered by the chemical modification procedures and diminished in intensity or abolished by digestion with neuraminidase. These results suggest that the cytoplasmic mucin is sialomucin without side-chain substituent in extramammary Paget's disease located in axillary or genital area, and with a substituent at C7 in the disease located in perianal area.