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FKBP23 was found in mouse endoplasmic reticulum (ER) in 1998. It consists of an N-terminal peptidyl-prolyl cis-trans isomerase (PPIase) domain and a C-terminal domain with Ca2+ binding sites. The fusion protein of mouse FKBP23 and glutathione S-transferase (GST), GST-FKBP23, and the fusion protein of BiP, a member of heat shock protein 70 (Hsp 70) in ER, and GST, GST-BiP, were subcloned in E. coli, expressed and purified. The fusion proteins were restrictively digested by Factor Xa (FaXa) to obtain the free cloned proteins FKBP23 and BiP. With the assay of adsorption of free FKBP23 or BiP with GST-BiP or GST-FKBP23 attached to the Glutathione-Sepharose 4B, the adsorbed FKBP23 or BiP could be detected by Immunoblot. It means that FKBP23 binds to BiP. Furthermore, BiP in leukocyte ER-extract can be adsorbed with GST-FKBP23 attached to the glutathione-Sepharose 4B. It shows that FKBP23 binds to natural BiP in ER, too. These experiments show that a PPIase binds to a molecular chaperone of the Hsp70 family.