Protein breakdown in the rumen often leads to excessive ammonia production and inefficient use of dietary protein by ruminants (Wallace et al., 1997). Attention has for many years focussed on the proteolytic activity of ruminal microorganisms (Wallace et al., 1997). The wide variety of proteolytic species and proteolytic enzymes and their between-animal variability has made the task of decreasing microbial proteolytic activity difficult (Falconer & Wallace 1999). Much less attention has been paid to the contribution of proteinases originating from the feed. In particular, grass cells contain vacuoles harbouring broad spectrum proteinases which are known to be responsible for protein breakdown in the silo (Wetherall et al., 1995). Theodorou et al. (1996) proposed that much of the rapid release of ammonia in grazing animals might be initiated by the action of plant, rather than microbial, proteinases. The present study was undertaken to compare the proteolytic activities of fresh grass and ruminal microorganisms and to evaluate their likely contributions to ammonia production in the rumen.