Denatured Clostridium tyrobutyricum flagellin is rapidly degraded during incubation with commercial preparations of alpha-amylase, dextranase, endo-1,3,(4)-beta-glucanase, chitinase and hyaluronidase indicating the presence of contaminating proteinases. Electrophoretic and solid-phase assay, with bovine serum albumin as substrate, revealed proteolytic activities in these preparations. In comparison to bovine serum albumin, the damaging of flagellin by proteolysis indicates that this protein would be useful as a general substrate in proteinase detection.