Commercial proteases yield more reproducible estimates of extent of rumen protein degradation than either in situ or rumen in vitro methods. Although extents of protein digestion determined using commercial proteases have been positively correlated to extents of in situ protein degradation, these values have not proven reliable predictors of rates of rumen protein degradation. This is because no protease-based assay has yet been developed that successfully mimics the specificities of the organisms involved in protein degradation in the rumen. If the proteolytic activity of rumen microbes and commercial proteases can be correctly characterised, then it would be possible to elaborate protease blends that match the protein degradative activity of mixed rumen microbes. The research reported here is an extension of earlier work as a prelude to preparing enzyme mixes with “protease fingerprints” similar to rumen microbes.