Six-day SRC assay as a chemosensitivity test has an advantage of high predictive rate for clinical response. However, it is pointed out that very few viable tumor cells are observed at the end of the assay, so that it may make the assay results unreliable. In this paper, we tested the effect of immunosuppressants on SRC assay using Walker carcinosarcoma originated from Wistar rat xenografted under the renal capsule of BDF1 mice. The changes of tumor size, pathological features and proliferative ability of xenografted tumor under the renal capsule of mice treated with cyclophosphamide, mizolibine or cyclosporin A are examined. Only cyclosporin A treatment could maintain the viable tumor cells and proliferative ability of the tumor grafted under the renal capsule 21 days after transplantation. In order to compare the original 6-day SRC assay developed by Bogden et al, we applied immunosuppressants to the 6-day assay. It is suggested that cyclosporin A and mizolibine amplify the sensitivity of tumor in 6-day SRC assay.