A chymotrypsinlike protease with an Mr of 95,000 was extracted from Treponema denticola ATCC 35405 and was partially purified by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The proteolytic activity was detected in an electrophoretogram containing polyacrylamide that was conjugated to bovine serum albumin. A single band of activity was detected when the T. denticola extract was solubilized and electrophoresed in the presence of sodium dodecyl sulfate. No activity was found in extracts of Treponema vincentii. The enzyme hydrolyzed transferrin, fibrinogen, alpha 1-antitrypsin, immunoglobulin A, immunoglobulin G, gelatin, bovine serum albumin, and a synthetic peptide containing phenylalanine. It did not degrade collagen or synthetic substrates containing arginine or proline. For the hydrolysis of azocoll, the pH optimum of the enzyme was 7.5. Heating at temperatures above 50 degrees C destroyed the activity. Reducing agents and the chelators EDTA and ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid increased the enzyme activity, while phenylmethylsulfonyl fluoride, L-1-tosylamide-2-phenylethyl chloromethyl ketone, sulfhydryl reagents, and human serum reduced activity. The ability of the enzyme to hydrolyze a number of humoral proteins suggests that it may be involved in spirochete invasiveness and tissue destruction.