Immunocytochemical screening of bladder tumors with an antibody to beta-human chorionic gonadotrophin (hCG) demonstrates a correlation with malignant behavior in that 0 of 20 G1M0, 5 of 20 G3M0, and 11 of 20 G2/3M + showed strong staining, while only 2/20 G3 Ca prostate and 0/18 renal cell carcinoma showed weak staining. There was a suggestion that hCG positive bladder cancer responded less well to chemotherapy (1 of 12 hCG positive and 6 of 13 hCG negative patients achieved a complete or partial response) and also patients whose tumors showed strong hCG staining did less well after radiotherapy, though paradoxically patients showing only occasional stained cells did better than patients whose tumor showed no staining. The possible significance of these observations in relation to recent observations on the mechanisms of trophoblast escape from immune surveillance is discussed.