ABO(H) isoantigen expression was estimated semiquantitatively with the red cell adherence test on single epithelial cell suspensions from 89 bladder tumor and 7 normal bladder specimens. Correlations of red cell adherence counts positive for antigen with the pathological findings of the bladder (histology and stage of tumor) were made in a blind fashion at the end of the study. The mean count positive for antigen from normal bladders was 86.0, which was significantly different (p less than 0.001) from those of muscle-invading lesions (12.6) and flat carcinoma in situ (21.6). This study shows a good correlation among the cell counts positive for antigen, histological findings and stage of disease, and provides the basis for a prospective study that may help to determine the role of ABO(H) isoantigen measurement in bladder tumor patients.