The structure of the unligated human T-cell recombinant cyclophilin has been determined at 3 A resolution by multipole isomorphous replacement methods and refined at 2.5 A resolution to an R factor of 0.209. The root-mean-square errors of the bond lengths and bond angles are 0.013 A and 2.8 degrees from ideal geometry, respectively. The overall structure is a beta-barrel, consisting of eight antiparallel beta-strands wrapping around the barrel surface and two alpha-helices sitting on the top and the bottom closing the barrel. Inside the barrel, seven aromatic and other hydrophobic residues form a compact hydrophobic core. A loop of Lys-118 to His-126 and four beta-strands (B3-B6) constitute a pocket we speculate to be the binding site of cyclosporin A.