R.J. WALLACE, S.J.A. WALLACE AND N. MCKAIN. 2000. Proteolytic activity was measured by the digestion of C-labelled casein in digesta removed from the rumen of four sheep receiving a grass hay/concentrate diet and four sheep receiving a maize silage/concentrate diet. Samples were removed immediately before feeding and at 2-h intervals after feeding up to 12 h. Animals on both diets produced similar proteolytic activities (1·83 (S.D. 0·41) and 2·14 (S.D. 0·61) mg C-casein hydrolysed (ml ruminal fluid) h with the maize silage- and grass hay-based diets, respectively). Time after feeding had no effect on proteolytic activity, but between-animal variation was consistent and highly significant, with the highest-activity animals having activities 64 and 74% higher than the lowest-activity animals on the two diets, respectively.