We have previously reported the leukocyte migration inhibition (LMI) factor in cancer patients, which is one of the specific parameters of cell mediated immunity. Since interpretation of these results was sometimes difficult in human cancer because of the unknown participation in the reaction, the inbred mouse strains were used as an experimental animal. We studied how the LMI factor of murine spleen cells changed with immunization of tumor cells, and intraperitoneal injection of protein polysaccharide (PSK) and/or cyclophosphamide (CPA). Method. LMI was performed by Clausen's method. Mouse strain; C3H/Heston. Tumors; C3M2 (methylcholanthrene induced tumor) and MAC (spontaneous mammary tumor). Result. Migration cells of murine spleen cells were almost granulocytes. Migration inhibition was maximum in 2 to 3 weeks after immunization of tumor cells. In vivo studies as well as in vitro studies revealed that C3M2 and MAC were not crossreactive. Migration was accelerated with CPA and inhibited with PSK. Leukocyte migration was inhibited by injection of PSK after treatment of CPA at a dose of 50 mg/kg, while in the case of CPA at a dose of 200 mg/kg, preinjection of PSK inhibited leukocyte migration.