Before the Clostridium tumour assay can be applied to the diagnosis of cancer, we sought to investigate--within the framework of a biopharmaceutical safety test--the organ persistence of test spores of Clostridium butyricum CNRZ 528. We found that non-pathogenic spores obtained in vitro, like pathogenic native spores, escape phagocytosis in various organs up until about 2 years, as tested by anaerobic cultures. The elimination of spores depended on the species of animal, the spore dose and the organs investigated. In rabbits, one week after injection, we recovered clostridial spores from blood and spleen cultures more rarely than from liver and lung. The half-life of blood clearance in patients was one day or, at half the spore dose, two days. That deep tissues of healthy animals are not normally sterile became evident in rabbits after sporadic isolation and characterization of non-administered saccharolytic and proteolytic clostridial species. During a 10-year observation period, the rate of obtainment of viable spores by in vitro cultures lessened; however, for administration of the spores in clinical phase I and phase II studies, the spore quality was acceptable.