CYCLOSPORIN A, a cyclic fungal undecapeptide produced by Tolypocladium inflatum, is a potent immunosuppressive drug originally isolated as an antifungal antibiotic^1,2. Cyclosporin A (CsA) is widely used in humans to prevent rejection of transplanted organs such as kidney, heart, bone marrow and liver³. The biochemical basis of CsA action is not known: its primary cellular target has been suggested to be calmodulin^4,5, the prolactin receptor^6,7 or cyclophilin, a CsA-binding protein originally isolated from the cytosol of bovine thymocytes^8–10. Cyclophilin has been shown to be a highly conserved protein present in all eukaryotic cells tested^11–15 and to be identical^16,17 to peptidyl-prolyl ds-trans isomerase^18,19, a novel type of enzyme²⁰ that accelerates the slow refolding phase of certain proteins in vitro^21,22. We demonstrate that in the lower eukaryotes N. crassa and S. cerevisiae, cyclophilin mediates the cytotoxic CsA effect. In CsA-resistant mutants of both organisms, the cyclophilin protein is either lost completely or, if present, has lost its ability to bind CsA.