Collagenase activity in human bladder carcinomas was measured against 14C-labeled collagen as substrate. Enzymes activity in vivo increased with the degree of penetration of the bladder wall. It was not detectable or low in the cases of superficially infiltrating tumours (A, B1 B2) and high in the cases of deeply infiltrating tumours (C, D). These results suggest that collagenase activity of advanced tumours is predominantly expressed in the pervesical layer of the bladder wall. A quantitative estimate of this enzyme may thus help to distinguish between Stage C and Stage C tumour.