L-Asparaginase from Cylindrocarpon obtusisporum MB-10 inhibits the growth of Ascites Fibrosarcoma and Dalton's Lymphoma tumor cells in vivo and significantly increases the survival rate of tumor bearing mice. The enzyme-treated normal mice become more healthy and survive longer than their usual life span. The spleen size of normal animals treated with L-asparaginase become larger, and the number of their rosetting T-lymphocytes along with the capacity of SRBC constellation gets increased. The surface topography of splenic T-lymphocytes of enzyme-treated mice exhibits some extensions of different parts of the membrane with ruffling of surface and formation of innumerable blebs, foldings, microvilli, etc. The adherence of leukocytes of peritoneal exudate cells of these mice is also enhanced. All results suggest that C. obtusisporum MB-10 L-asparaginase is active against tumors and non-immunosuppressive, and it deserves to be an immunotherapeutic agent.