The ingestion of tannins and other polyphenols by ruminants can adversely affect the growth and fibre-digesting activities of the rumen microorganisms (Muhammed et al. 1994). However, components of rumen liquor such as preformed monomers (amino acids, purines and pyrimidines) and other nutrients may protect the microorganisms, by providing nutritional conditions optimal for growth and energy metabolism. Rumen liquor also contains plant proteins, to which phenolic substances may bind preferentially. The influence of some polyphenols on the degradation of cellulose by the anaerobic fungus Neocallimastix frontalis strain RE1 has been studied in two nutrient media considered to embrace the range of nutritional conditions that may occur in the rumen at different times. The media used were medium M2 of Hobson (1969) which contains rumen fluid casein hydrolysate and yeast extract, and the medium of Hungate and Stack (1982) (medium HS) which is chemically defined and in particular lacks protein hydrolysate and rumen fluid. Whatman no. 1 filter paper cellulose (30 mg) was the growth substrate. Filter-sterilized solutions of tannic acid (dissolved in dist. H20) or ellagic acid, gallic acid or catechin (dissolved in dimethylsulphoxide, DMSO) were added to the media (10 ml) prior to inoculation with strain REl (approx. 104 zoospores) and incubation for 5 d at 38°C. Separate control incubations were performed with the 2 solvents (H20 and DMSO) used. The compounds tested were more inhibitory towards fungi grown in the defined medium (HS) than in the rumen fluid-containing medium M2 (Table 1). Thus tannic acid reduced cellulolysis to around 50 % of the relevant control values at a concentration of approx. 0.1 mM in HS, and 0.8 mM in M2. Corresponding values for ellagic acid were 0.15 mM and 0.5 mM, for gallic acid 0.25 mM and >3.6 mM and for catechin 0.15 mM and >2.4 mM respectively. It seems that these phenolic compounds are potent inhibitors of cellulolysis by Neocallimastix by mechanisms yet to be elucidated. It also appears that proteinaceous media reduce the inhibitory effects, suggesting that interactions occur between the proteins in the medium and the test compounds. It seems that in the rumen, dietary proteins and/or peptides may partly protect the fungi from the effects of polyphenolics as seen here in vitro.