Freshly resected human tumors were grown as xenografts under the kidney capsules of conventional mice immunosuppressed by daily treatment with cyclosporine A (CsA; 60 mg/kg/day, sc). Tumor persistence and growth in CsA-treated mice were comparable to those found in nude mice. Lower doses or less aggressive schedules of CsA resulted in xenograft rejection. CsA (60 mg/kg/day, sc) treatment of mice was toxic but not lethal. It resulted in a 20% decrease in water consumption, a 10% loss in body weight, and significant reductions in thymus, kidney, and liver weights. Therapeutic doses of cancer chemotherapeutic agents were tolerated by CsA-treated mice, but 10% lethal doses were lethal in 100% of the mice. These results indicate that CsA-treated mice would be an economic alternative to nude mice for growing human tumor xenografts and for testing their chemosensitivity, with the caveat that the dose-limiting toxicity of cancer chemotherapeutic agents will be reached at lower doses in CsA-treated mice.