We have previously reported the production of monoclonal antibody (MAb) IBEI2, which recognizes a glycoprotein in breast-cancer cells. In the present work, I BE1 2 reactivity was tested by immunohistochemistry in bladder carcinoma (92 cases) and in non-tumoral bladder samples (I 5 cases). In 7 I % of bladder tumors, more than 30% of cells were intensely stained by IBEIZ. The percentage of reactive cells was higher in cancers invading the muscle than in more superficial tumors (p = 0.039). In non-tumoral bladder, immuno-staining, when present, was usually confined to the superficial layers with a low number of cells stained (<30%) in 13/15 cases. Slot blots, performed on urine samples from 43 bladder-cancer patients and 21 healthy controls, were quantified by densitometry scanning, We found higher optical density (OD) values in urine from muscle-invasive-cancer patients than in urine from more superficial tumors and healthy controls, with a significantly different distribution (p = 0.005). The urinary antigen was detected by immunoblotting with I BE I2 as highmolecular-weight species (> I50 kDa). The reactive glycoprotein could thus be purified by immunoaffinity and FPLC filtration from the perchloric-acid-soluble fraction of urine from patients with invasive bladder carcinoma. The availability of purified antigen will allow us to quantitate our assay, in order to evaluate its potential use as a prognostic indicator in bladder-cancer patients.