The effect of active immunotherapy with Vibrio cholerae neuraminidase-treated syngeneic tumor cells (VCN-cells) following radiotherapy has been studied with 3-methylcholanthrene-induced fibrosarcoma, M-79, transplanted to the thigh of C3H/HeJ mice. When the tumors reached 4-8 mm in diameter, various treatments were started. X-irradiation with 2000 rad in a single dose induced a complete regression of 24 out of 103 tumors (23.3%). The inoculation of 1× 106 of VCN-cells to the tumor-bearing animals, every other day for a total of three doses, caused a complete regression of 6 out of 57 tumors (10.5%). Treatments of animals with the immunotherapy starting 1 day after X-irradiation of tumors with 2000 rad resulted in a complete regression of 22 out of 58 tumors (37.9%). The median survival time of animals that received combined radiotherapy and immunotherapy was longer than that observed after either treatment alone.

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