Experimental evaluation of a new chemosensitivity test using a subrenal capsule assays (SRC) was performed. The effects of various immunosuppressants-cyclosporin A (CSA), cyclophosphamide, whole body irradiation, and Bredinin were studied using human small cell lung cancer cell line (SBC-3) serially transplanted in nude mice. A significant degree of host cell infiltration was seen in tumor fragments implanted under the renal capsule of immunocompetent mice. However, treatment with immunosuppressants effectively suppressed the host immune reaction. The most effective immunosuppressant was CSA at 60mg/kg. We compared the antitumor activities of CDDP, MMC, VP-16, ADM, CPA, and VCR against SBC-3 using SRC and clonogenic assays. SRC was performed using mice administered CSA 60mg/kg. Chemotherapeutic agents (1/2 LD₁₀) were administered on day 1 and antitumor activities were evaluated on day 6 after implantation. The results of the assays were well-correlated except with VCR.