Two incubation runs were conducted with Rusitec fermenters to investigate the effects of three additive treatments (mixed fibrolytic enzymes from Trichoderma longibrachiatum (FE), disodium fumarate (FUM) and both additives (MIX)) on rumen microbial growth and fermentation of a grass hay:concentrate (600 : 400 g/kg DM) substrate. Each fermenter received daily 20 g substrate DM. Application rate (per g substrate DM) was 34·3 endoglucanase, 0·57 exoglucanase, 24·7 xylanase and 5·51 amylase units for FE and 30 mg fumarate for FUM. MIX fermenters received both additives. Both FE and MIX increased (P < 0·05) daily production of acetate, butyrate and methane, substrate DM and fibre disappearance at 6 and 48 h incubation, daily flow of microbial-N, and the microbial colonisation of substrate at 6 h incubation. Compared to FE, MIX treatment increased (P < 0·05) propionate production by 28 % and decreased (P < 0·05) the acetate:propionate ratio, but no other differences between both treatments were found (P>0·05). Supplementing with FUM increased (P < 0·05) volatile fatty acid production by 11 % and decreased (P < 0·05) the acetate:propionate ratio, but did not affect (P>0·05) any other variable, thus suggesting that observed effects were due to fermentation of FUM itself. The lack of effects of FUM and the absence of differences between FE and MIX on most of the measured variables would indicate that beneficial effects found in MIX fermenters were mainly due to the action of FE. Combining FE and FUM as feed additives under the conditions of the present experiment did not further improve rumen fermentation, compared to FE alone.