We studied whether or not cyclosporin A (CSA) has a usefulness in subrenal capsule assay (SRCA) with normal immunocompetent mice. Sixty mg/kg of CSA was given to BDF1 mice daily subcutaneously, and various dosages of adriamycin (ADR) was given intravenously on day 2. The body weight of BDF1 mice decreased over 20% within ten days when ADR was given at more than 5 mg/kg. MX-1, a human breast carcinoma line is known to be sensitive to ADR. This tumor was implanted subcutaneously in the back of BALB/c nu/nu mice and chemosensitivity was tested against ADR. ADR resulted to be positive at the dose of 8 mg/kg. On the contrary, the dose of 5 mg/kg proved to be negative, and hence the result of SRCA would be false negative, if the dose of ADR is reduced to avoid the toxicity of CSA. The tumor grew slowly when only 60 mg/kg of CSA was given daily for three weeks, and the inhibition rate was 56.2%. The toxicity of CSA was neglected because of the body weight loss was approximately 13%. CSA may have the antitumor effect by itself, and we therefore suggest that the CSA is not useful for SRCA.