ABH isoantigens in 82 cases with bladder tumor of various grades and stages were investigated by the avidin-biotin-peroxidase complex (ABC) method. Correlation of ABC expression for isoantigens with pathological findings on the grading and staging system of the bladder tumors were as follow: Of 60 cases of low grade tumors, 34 (56.7%) were positive and of 22 cases of high grade tumors, 21 (95.5%) were negative. Of 63 cases of low stage tumors, 35 (55.6%) were positive and all 19 cases of high stage tumors, (100%) were negative. The isoantigens in the tissue of the bladder tumors tended to disappear as the histologic anaplasia of the tumor progressed. There was a high correlation between the histological grade, stage and ABC expression (p less than 0.01). In a follow-up study of 38 patients with low grade, low stage, initial transitional cell carcinoma, 9 of the 24 patients positive for ABC expression (37.5%) showed recurrence during the 1 to 6 year follow-up period, whereas 10 of the 14 patients negative for ABC expression 10 (71.4%) did. There was a high correlation between the recurrence rate and ABC expression (p less than 0.05). Therefore, the analysis in the bladder tumors may be valuable for prediction of malignant potential, especially for the low grade, low stage tumors.