1. A method was developed for extracting enzymes from micro-organisms closely associated with ammonia-treated straw (NH-S) that had been incubated in nylon bags in the rumen. Incubation of washed straw with 125 ml carbon tetrachloride/l and 20 µg lysozyme/ml for 3 h at 37° gave carboxymethylcellulase (EC 220.127.116.11; CMCase) and NAD-linked glutamate dehydrogenase (EC 18.104.22.168; GDH) activities greater than those extracted by sonication. 2. GDH associated with NH-S increased with incubation time and was highest in sheep receiving a high-barley diet. Particle-bound CMCase activity reached a peak between 16 and 24 h and declined thereafter. 3. Particle-bound GDH activity showed no correlation with dry matter (DM) degradation in the rumens of sheep fed on a range of diets. In contrast, CMCase activity after 24 h was highly correlated with DM degradability of the same samples at 24 h (r 0·98) and 48 h (r 0·94). 4. It was concluded that GDH and CMCase can be used as indices of the total population of colonizing rumen micro-organisms and of the fibre-degrading population respectively, and that these enzymes can therefore be used to assess rapidly and with great sensitivity variations in the rumen environment that affect the rate of fibre breakdown.