Human group Ⅹ phospholipase A₂ is a member of mammalian secretory phospholipase which belongs to phospholipase A₂ superfamily. It has been expressed in a form of inclusion bodies in E.coli. It could not refold efficiently as human pancreatic phospholipase A₂ (group Ⅰ B) merely by diluting the protein unfolded in 8 mol/L of urea. The refolding reaction of human group Ⅹ phospholipase A₂ in vitro was depended on temperature, pH and protein concentration. A number of additives have been tested, among which L-arginine was the most efficient effector in improving the refolding of human group Ⅹ phospholipase A₂, and its structural analogs, L-citrulline, had less effect. L-lysine and L-arginine methyl ester could not improve phospholipase A₂ refolding. In addition, L-arginine inhibited the formation of aggregates and of intramolecular disulfide bonds. These results showed that both carboxyl and guanidyl residues of L-arginine were essential for improving protein refolding.