The mechanism by which enzyme additives improve feed digestion in ruminants is not fully understood. Direct hydrolysis of feed in the rumen is a potential mode of action, but the importance of this mode needs to be quantified because of the relatively low exogenous hydrolase activity added compared with the total activity present in the rumen. We examined the interactions between ruminal and exogenous enzymes on fiber degradation using a completely randomized experimental design, with an 11 (enzyme preparations and their combinations) x 5 (assay pH) arrangement of treatments. Ruminal enzymes were extracted from cattle receiving high fiber or high concentrate diets and exogenous enzymes were Trichoderma longibrachiatum preparations containing different proportions of xylanase and cellulase activities. Ruminal and exogenous enzyme preparations and their combinations were tested for the ability to degrade soluble cellulose, xylan, and corn silage over a range of pH from 4.5 to 6.5 at 39 degrees C. T. longibrachiatum enzymes acted synergistically with enzymes from mixed rumen microorganisms in degrading soluble cellulose, xylan, and corn silage. Hydrolysis increased by up to 35, 100, and 40% for soluble cellulose, xylan, and corn silage, respectively, and was most evident at a pH range between 5.0 and 6.0. The synergistic effect between ruminal and exogenous enzymes increases the hydrolytic potential within the rumen environment and is likely a significant mechanism by which enzyme additives improve feed digestion.