Summary— Data are presented on a group of cases of primary carcinoma of the bladder detailing their ABO, MNSs and rhesus blood antigenic phenotypes. Differences, either between all cases and controls or between particular stages of presentation and controls, have been found in the ABO red cell system as well as the MNSs red cell system. In the case of the ABO system an excess of the A gene is present but detailed study shows this to be only in patients with T1 and T2 tumours. No excess was seen in patients with carcinoma in situor T3 or T4 stage disease. The S gene of the MNSs system is also in excess in cases of bladder cancer.