Rumen fluid was taken from fistulated sheep and a cow receiving various diets based on grass hay or grass cubes with and without cereal-based concentrates. Proteinases in the extracellular fluid, and extracted from particulate material by Triton X-100, were visualized using SDS-PAGE in which gelatin was co-polymerized with the gels. Each animal sampled had a different pattern of proteinase activity. No single proteinase band predominated, although a few appeared in several samples, indicating that some microbial species were commonly involved in proteolysis but none was dominant. The banding patterns in samples taken from the same animals two weeks apart were fairly similar, indicating some stability within animals. Patterns obtained with extracellular fluid and Triton X100 extracts of small and large particulate material from the same sample were similar for the most part, although the relative intensity of the bands differed. The serine protease inhibitor, phenyl methyl sulphonyl fluoride, had little influence on the banding pattern. Concentrate in the diet appeared to increase inter-animal variation, and sheep in adjacent pens and consuming the same grass hay:concentrate diet had different banding patterns. Thus, no microbial proteolytic enzyme was predominant in protein digestion in the rumen.