An improved purification of the cathepsins B1 and B2 from bovine spleen is described. In addition to the formerly used procedure, chromatography with DEAE-Sephadex or -cellulose and mercurated agarose is used. Both enzymes are obtained in an electrophoretically pure form but consist of two or more isoenzymes. The isolation procedure leads to enzymes with high specific activities in satisfactory yields. Cathepsin B1 is frequently accompanied by small amounts of an arylamidase-like enzyme that hydrolyzes leucine p-nitroanilide. However, very probably, cathepsin B1 itself has a low activity toward this substrate too. Cathepsin B2 has a comparatively high activity with its characteristic though not specific substrate, alpha-N-benzoyl-L-arginineamide, whereas the activity toward haemoglobin is far lower. Both enzymes possess an essential SH group and require EDTA and a mercaptane for full activity, but their stability is markedly impaired by storage at higher thiol concentrations; Some other properties of the enzymes are also discussed.