Peptidyl prolyl cis/trans isomerases (PPIases) catalyze the slow cis/trans isomeraization of proline peptide (Xaa-Pro) bonds in oligopeptides and accelerates slow, rate-limiting steps in the folding of several proteins. We studied the characterization of Cyclophilin A (bCyp A) isolated from Bombyx mori . The cDNA of bCyp A is 947 bp. There is a 5´-untranslated region of 91 nucleotides followed by an initiating ATG codon. The TAA termination codon occurs at nucleotide 588. Thus translation of the sequence from nucleotides 91 to 588 would produce a protein of 166 amino acids with a calculated molecular mass of 18.2kDa. The 'AATAAA' consensus polyadenylation signal and poly A tail are present in the 3´-untranslated region. To analysis of PPIase activity, we expressed the bCyp A protein in Sf9 cell by using baculovirus expression vector system (BEVS). SDS-PAGE and Western blot analysis showed that the molecular weights of intracelluar expressed protein was approximately 28.2 kDa. The PPIase activity assay was monitored by proteolytic cleavage of the chromophore p-nitroanilide byα-chymotrypsin. As substrate the synthetic tetrapeptide succinyl-Ala-Ala- Pro-Phe-p-nitroanilide was used.