We have identified an 8.4 kDa minor immunophilin from calf thymus and Jurkat T cells as ubiquitin. It binds tacrolimus and sirolimus with K-d's of 0.8 and 0.08 nM, respectively, The binding of this protein to cyclosporin A is negligible. Binding of tacrolimus to two commercial sources of ubiquitin, a bovine product, and a recombinant human ubiquitin, was also demonstrated after HPLC purification of the purchased preparations.