Background: Ciclosporin A (CS-A) is a powerful immunosuppressive agent, isolated from the fungus Tolypocladium inflatum Gams. The value of CS-A in experimental oncology is at present not clear. Studies on xenogeneic tumor transplantation, on the reversal of cytostatic resistance in various tumor models, and on the combination of CS-A with different other cytostatic drugs have been published. Materials and Methods: The 6-day subrenal capsule assay (SRCA) was used. A murine reticulum cell sarcoma was transplanted under the renal capsule of female BlmNMRI mice. Different doses of CS-A (100, 80, 60, 40, and 10 mg/kg) were administered on 6 consecutive days both intraperitoneally and subcutaneously. Tumor size, body weight reduction, and mortality were evaluated.Results: The data demonstrate a significant cytostatic effect of CS-A in the murine reticulum cell sarcoma both after subcutaneous and intraperitoneal administration of CS-A at different doses. A dose-dependent tumor size reduction was found only in the subcutaneously treated animals. The toxicity, measured by body weight reduction, was also significantly higher in the subcutaneously treated animals, but the mortality was higher in the animals treated intraperitoneally with CS-A. Histologic eval-uation showed a reduction of the mitotic activity in all treated versus control groups.Conclusions: The results suggest that CS-A has a cytostatic effect in the murine reticulum cell sarcoma RS-180, transplanted under the subrenal capsule of syngeneic female BlmNMRI mice for 6 days, which is combined with a decrease in tumor size. Histologically, a reduction of mitosis in the groups treated with CS-A was found.