The dye 4-(4-aminophenylazo)phenylarsonic acid was coupled to activated CH-Sepharose 4B and the resulting affinity matrix was shown to be highly efficient for the purification of subtilisin. A crystalline subtilisin was purified to homogeneity using this affinity chromatography procedure with a purification fold of 1.4 and with an enzyme activity yield of 98%. Similarly subtilisin from a crude enzyme preparation was purified to 211 fold by this single step procedure with 94% recovery of the enzyme activity. The purified enzymes were shown to be homogeneous by polyacrylamide gel electrophoresis.