The effects of an enzyme preparation on the performance and carcass characteristics of broilers fed diets based on maize, wheat, hulled barley (Bedford) or micronized dehulled barley (MDB) (Bedford) diets were evaluated. A commercial enzyme containing (3-glucanase and xylanase was used. One thousand and nine hundred-twenty one-day-old broiler were used for this study. The broilers fed the MDB diets had a lower average daily gain (ADG) (P<0.05) and lower average daily feed intake (ADFI) (P<0.05) than the broilers fed the other diets. There was a significant period x grain interaction (P<0.05) for ADG with the broilers on the barley diets performing better in the grower phase. The broilers received enzymes showed an enzyme x period interaction with the feed conversion rate (FCR) of those birds in the starter phase doing better (P<0.05) than in the grower phase compared to those fed the unsupplemented diets. The birds fed MDB diets had higher digesta viscosity than birds fed unprocessed barley, maize or wheat diets (P<0.01). Feed enzyme addition to the MDB diet caused a decrease (P<0.01) in the digesta viscosity by 49% and an improvement (P<0.05) feed conversion ratio (FCR) both for the starter and the grower phase.