Effect of salt concentration on the activity and stability of cathepsin B from buffalo spleen has been investigated. Catheptic activity was maximal at a buffer concentration of 5.5 x 10(-3) M. The enzyme was, however, highly unstable at a salt concentration lower than 1.5 x 10(-2) M. The use of 0.02 M sodium phosphate buffer has been suggested for the assay of cathepsin B activity. For storage of the enzyme, however, a 0.1 M sodium phosphate or other buffer of equivalent ionic strength has been recommended.