Anti-Leu-M1 antibody, which is well known to react with a human myelomonocytic antigen, was found to react with bladder carcinoma, but never with the normal bladder epithelium. Eighty-six cases of transitional cell carcinoma of the bladder were tested in sections of formalin-fixed and paraffin-embedded tissues by using an avidin-biotin immunoperoxidase technique. Fifty per cent of these cases were positive in Leu-M1, while the other three monoclonal antibodies (HBA4, HBG9 and HBH8), which were produced against a human bladder carcinoma cell line KU-1, reacted in 17 per cent, 27 per cent and 64 per cent of these cases respectively. Reactivities of these antibodies to 33 cases of normal bladder epithelium were Leu-M1, 0 per cent; HBA4, 0 per cent; HBG9, 0 per cent and HBH8, 30 per cent, respectively. Expression of Leu-M1 antigens in bladder epithelium seems to be specific for malignant change and highly frequent in the carcinoma patients. The relation between Leu-M1 expression and a degree or stage in bladder carcinomas, however, was not significant in our studies.
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