A test system for the evaluation of ABH blood group isoantigens in bladder carcinoma was developed. A highly standardized staining technique and a set of criteria for the judgement of staining extent and intensity was used. These variables are given numerical values which are multiplied to give a score (0-12) representing a semiquantitative estimate of the ABH-antigen expression. A consecutive series of 230 patients with bladder carcinoma was analyzed, using the test. The correlation between staining scores on the one hand and the tumor grade and stage on the other was assessed. Thirty-three percent of the tumors, irrespective of grade or stage, were completely negative, whereas 15% were positive to the same extent and intensity as normal mucosa. The remaining tumors showed intermediate patterns with respect to both extent and intensity of the expression with a progressive loss of detectable antigens with increasing grade and stage. The material was categorized with respect to the staining patterns for future analysis of the influence on prognostic parameters.