The intravenous injection of (20–40) × 106heat-treated spores ofClostridiumM55 into Swiss mice bearing a 10 × 10 mm Ehrlich solid tumor was followed in about 15 days by regression of the tumor in 3 out of 10 animals. Histological sections confirmed the absence of tumor cells. This result was obtained with only two lots of spores; on the other hand, four additional lots cured 2 tumors out of 10. Since growth ofClostridiumM55 in cellophane tubes shows that the active principle does not dialyze but may occasionally diffuse into the surrounding culture fluid, it would appear interesting to study the conditions which permit this liberation with the hope that it may be extracted, purified, and identified. In conclusion, it would appear that this is the first time that a biological material causes regression of Ehrlich's solid tumor in the mouse.