Sanglifehrin A (SFA) is a recently discovered immunosuppressant drug that shares its intracellular target with the major immunosuppressant drug cyclosporin A (CsA). Both bind to and inhibit the cyclophilins, a diverse family of proteins found throughout nature that share a conserved catalytic domain. Although they share this common protein target, the mechanism of action of the cyclophilin-SFA complex has been reported as distinct from that of the well-studied cyclophilin-CsA complex. The X-ray structure of a macrolide analogue of SFA's cyclic region complexed with cyclophilin A has recently been resolved, but this left the placement of the linear region of SFA unresolved. Using five cyclophilins from the fission yeast Schizosaccharomyces pombe, and a mutant of one of these proteins, SpCyp3-F128W, we have shown that the sensitivity of cyclophilins to SFA can be correlated to the same specific tryptophan residue that has previously been identified to correlate to CsA sensitivity, and that the tail of SFA may be responsible for mediating this sensitivity.