Intravenous injections of suspensions of spores of Cl. butyricum (Strain M-55) produced a more or less extensive lysis of tumor tissue. A number of other strains of nonpathogenic clostridia did not show substantially improved results. In genreal, strains which were able to ferment carbohydrate performed equally well, whereas primarily proteolytic strains gave inferior results. Combination of Strain M-55 with Cl. felsineum, as well as a combination of Strain M-55 with culture filtrates of Cl. paraputrificum, did not yield improved results. All growth phases of Cl. butyricum could at least in principle produce the phenomenon of oncolysis, but the results were most unfavorable if vegetative forms were used. To some extent an enhancement of oncolysis could be obtained by a combination of spores with certain tumor extracts.