Since the discovery of the "Clostridium tetani phenomenon", various apathogenic clostridia have been used for tumour lysis. Experiments have been conducted to achieve a tumour diagnosis using radiolabelled antibodies to clostridia. In addition, a method has been described that distinguishes, with variable success, between healthy and tumour-carrying animals by means of hemagglutination. The method outlined here uses the fact that malignant cells produce a multitude of sialic acid compounds which lie on the cell membrane and are also connected to the lipid layer of the tumour cell membrane. The apathogenic Clostridium oncolyticum M55 only germinates and multiplies in the malignant tumour tissue. Thus; bacterial hydrolases can enter the tumour tissue and lead to oncolysis. Subsequently the glycocompounds which can be detected by means of an enzymatic determination of the concentration of neuraminic acid (one of the sialic acids) in the serum are washed out into the peripheral blood. We observed these processes in mice in the Ehrlich ascites solid carcinoma and in the Lewis lung carcinoma. Using this method it was possible to detect tumour growth at an early stage with impressive accuracy. The Lewis lung carcinoma which secretes only small amounts of sialic acid glycocompounds cannot be distinguished from the control group by determination of sialic acid concentration. It was possible to detect a 52% increase in the amount of sialic acid after administration of spores of clostridia. This method makes it possible to increase the tumour marker sialic acid through manipulation of the tumour, using apathogenic clostridia, and to measure of sialic acid concentration as an indicator of the metabolic products of the tumour.