1. The rate of proteolysis in the rumen was dependent on the soluble protein used. With a sheep on a hay + concentrate diet the rates (approximately V",) for casein, Fraction I leaf protein and bovine serum albumin were 163, 3.4 and 0.9 mg protein nitrogen/l per min respectively. 2. Change of diet from hay+concentrate to fresh lucerne (Medicago sativa) increased the proteolytic rates for all three proteins. 3. Bovine submaxillary mucoprotein degraded extremely slowly in the rumen at

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... y 0.5-0.6 mg N/I per min and its sialic acid component was degraded at a similar rate to that of the protein chain. 4. Uniform1y1*C-labelled Fraction I leaf protein was used to demonstrate that in the presence of a second protein, competition for enzymic sites occurred. In Fraction I and bovine serum albumin mixtures, reduced rates for the individual proteins of the mixture were observed compared with the proteins treated separately. 5. Treatment of bovine serum albumin with dithiothreitol (0.2 g/l) to cleave disulphide bridges increased the rates of proteolysis by as much as 8.5-fold.