Cimetidine was given intravenously in daily dose of 20mg/kg to 20 patients with malignant tumor from the operative day on. The results show that cimetidine can elevate CD4, CD4/CD8 and decrease CD8. For comparison, blood samples from 40 patients were taken before operation for extracorporal testing. The results show that cimetidine can elevate lymphocyte transformation test and interleukin-2 secretive cell. These indicate that cimetidine can enhance the immune function of the patient and can be used as an adjunctive treatment of neoplasm.