It has been previously proposed that the rabbit p59-HBI (Heat shock protein Binding Immunophilin) or rFKBP59 (FK506 Binding Protein), found associated with the 90 kDa heat shock protein in nontransformed steroid receptor complexes, has three domains structurally related to hFKBP12 (Callebaut, I., Renoir, J.M., Lebeau, M.C., Massol, N., Burny, A., Baulieu, E.E. and Mornon, J.P. (1992) Proc. Natl. Acad. Sci., USA 89, 6270-6274). Here we report the overexpression, as fusion proteins in E. coli, of the full length p59-HBI and a series of p59-HBI mutants delimiting these domains and their respective peptidyl prolyl cis trans isomerase (PPlase) activity. The PPlase activity of p59-HBI is comparable to that of hFKBP12 and is due to domain p59-HBI I which displays the highest homology with this immunophilin. The residual enzymatic activity found in domain p59-HBI II is discussed.