AIM:To investigate the effectiveness of Clostridium novyi(C.novyi)-NT spores for the treatment of established subcutaneous pancreatic tumor in the syngeneic,immunocompetent Panc02/C57Bl/6 model. METHODS:C.novyi-NT spores were applied intravenously to animals carrying established pancreatic tumors of three different sizes.Systemic immune responses in peripheral blood and spleen were examined by flow cytometry.Supplementary,cytotoxic activity of lymphocytes against syngeneic tumor targets was analyzed. RESULTS:Application of spores identified,that(1) small tumors(<150 mm 3 )were completely unaffected (n=10);(2)very large tumors(>450 mm3)responded with substantial necrosis followed by shrinkage and significant lethality most likely due to tumor lysis syndrome (n=6);and(3)an optimal treatment window exists for tumors of approximately 250 mm 3 (n=21).In this latter group,all tumor-bearing animals had complete tu-mor regression and remained free of tumor recurrence. In subsequent tumor rechallenge experiments a significant delay in tumor growth compared to the initial tumor cell inoculation was observed(tumor volume at day 28:197.8±87.3 mm 3 vs 500.1±50.9 mm3,P<0.05). These effects were accompanied by systemic activation of immune response mechanisms predominantly mediated by the innate arm of the immune system. CONCLUSION:The observed complete tumor regression is encouraging and shows that immunotherapy with C.novyi-NT is an interesting strategy for the treatment of pancreatic carcinomas of defined sizes.