AbstractProphylactic and therapeutic effectiveness of killed C. granulosum and C. parvum bacteria was investigated against a methylcholanthrene‐induced fibrosarcoma in syngeneic C3Hf/Bu mice. Subcutaneous or intravenous treatment of mice with 0.25 mg of these bacteria greatly reduced the number of tumor nodules (metastases, colonies) in the lung generated by 105 or 106 fibrosarcoma cells inoculated intravenously 7 days later. The treatment also prolonged the survival of tumor‐cell recipients, and to some mice afforded complete protection against tumor growth. Number of lung metastases and survival of the recipients were more affected by intravenous than by subcutaneous treatment with the bacteria. Given intravenously to mice 3 days after intravenous inoculation of 2×105 fibrosarcoma cells, these non‐specific immunostimulants significantly reduced the number of pulmonary colonies and prolonged survival of mice. In contrast, subcutaneous application of the bacteria was only slightly effective. Both C. granulosum and C. parvum administered intravenously to mice 3 days following subcutaneous injection of 4×105 fibrosarcoma cells did not affect the development of subcutaneous tumors. However, when the tumors had grown to 25‐17 mm in diameter 67% and 80% of them underwent complete and lasting regressions in mice treated with C. granulosum and C. parvum, respectively. C. granulosum and C. parvum were approximately equally effective against both intravenously and subcutaneously injected tumor cells.