Kiwifruit (Actinidia chinensis) contains abundant protease, actinidin, and two possible components which were named Al and A2. However, a comparison of the two components has not been thoroughly conducted. We have previously shown the presence of six proteases named KPI, KP2, KP3, KP4, KP5 and KP6 in kiwifruit, and that each purified kiwifruit protease was chromatographically pure. It was also indicated that the two representative components, KP4 and KP6, must be Al and A2. To establish whether or not the two proteases, KP4 and KP6, have the same specificity in proteolytic activity, their enzymatic properties were compared. Between the two proteases, differences in substrate specificity against several protein-substrates (casein, gelatin, collagen, ovalbumin and bovine serum albumin) were not observed by digestion-product analysis with sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The kinetic parameters of KP4 against N-α-carbobenzoxyl-lysinep-nitrophenyl esters were different from those of KP6. The pH-activity profiles of KP4 and KP6 against S-3-trimethylaminopropyl-lysozyme, a wide-pH range soluble substrate, and N-α-carbobenzoxyl-lysine p-nitrophenyl esters were different.