Protease enzymes, produced by Bacteroides nodosus strains isolated from animals with virulent and benign forms of ovine footrot, were partially purified by ultra-filtration, ion exchange chromatography and gel permeation chromatography. Each enzyme had a similar pH optimum, was inhibited by phenylmethylsulfonyl fluoride (PMSF), ethylene diamine tetraacetic acid (EDTA) and ethyleneglycot-bis-aminoethylether-N,N-tetraacetic acid (EGTA), but was not inhibited by 1,10-phenanthroline. The results suggest that these enzymes are serine proteases that require divalent cations for activity. The enzymes could be distinguised by their differential temperature stability and differing susceptibility to irreversible inactivation by EDTA. Both enzymes were stabilised by incubation in the presence of Ca2+, but the enzyme purified from the virulent isolate required less Ca2+ for maximum stability. These results suggest that the differential thermostability of the protease activity detected in virulence tests is an intrinsic property of the protease enzymes.