Cathepsin L could not be removed completely during conventional actomyosin extraction and still remained in the actomyosin of red bulleye surimi. Sepharose 6B gel filtration profile showed that the main peak of cathepsin L was separated from that of actomyosin suggesting the enzyme was non-binding to actomyosin. The fractions showing the main activity of cathepsin L were pooled and mentioned as L(mix). Optimal pH of cathepsin L in actomyosin and L(mix) was 5.0 and the optimum temperature of L(mix) was 45 degrees C. Stability of L(mix) was closely related to temperature and pH. At optimum temperature 45 degrees C and optimum pH 5.0, activity of cathepsin L remained 76.8% after 120 min. At acidic pH 4.0, it remained 25% of its original activity after incubation at 45 degrees C for 120 min. At neutral pH 7.0, incubated at 45 degrees C, cathepsin L decreased the activity to 50% within 30 min and remained only 3% after 120 min. When incubated at low temperature 25 degrees C, cathepsin L kept 83-85% of its original activity at pH 4.0, 5.0 and 7.0. Effect of NaCl concentration on cathepsin L was related to pH. At neutral pH 7.0, in 0.6 M NaCl solution, cathepsin L activity was about 85% that of pH 5.0, indicating that cathepsin L might contribution to the gel deterioration of red bulleye surimi even in neutral condition. Studies of substrate specificity and effects of activators and inhibitors confirmed L(mix) to be a thiol-type cysteine protease.