The cytoplasmic sialomucin in Paget cells of both mammary and extramammary Paget's diseases was examined, using a new method proposed by Volz et al. (1987a, b). The staining methods used involved an electrolyte-Alcian Blue (pH = 5.8) and periodic acid Schiff. Oxidation was performed with 0.4 mmol/l periodate in 1 mol/l HCl at 4 degrees C or 50 mmol/l periodate in distilled water at room temperature for 1 h. Methylation, saponification, borohydride reduction, 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 in Paget cells is sialomucin without side-chain substituent in genital Paget's disease, and that with a substituent at C7 in mammary Paget's disease.