Three 4x2 factorial experiments were carried out to reevaluate the response of Hubbard broiler chicks to feeding on isocaloric (ME; 3100 kcal!kg) and isonitrogenous (about 19% CP) diets; containing graded levels [0.0 (control). 15, 20 or 25%] of each of cereal grain byproducts (CGB), namely rice bran (RB) in experiment 1, wheat bran (WB) in experiment 2 or their combination (RWB) in experiment 3, with or without exogenous enzyme (EA) mixture [Phytase, 150 g/ton and Avizyme-1500 (xyianase, amylase and protease), 1.5 kg/ton), from 2 to 6 weeks of age. Criteria of response were the performance of chicks for body weight gain (BWG), feed intake (FI) and feed conversion (FC), economic efflcency (EEF), nutrients digestibility, some carcass traits, and certain blood plasma constituents (glucose, total protein, total lipids, cholesterol, Ca and inorganic P). The results obtained can be summarized as follows: Regardless of dietary EA, increasing dietary level of RB UP to 20 or 25% significantly increased FI and decreased FC; digestibilities of DM and 0M also increased with the highest inclusion level of RB, whereas all other criteria were not affected. Feeding diets containing up to 25% WB, however, significantly improved EE digestibility but negatively affected EEF, all other measurements were not affected. On the other hand, feeding up to 20 or 25% of RWB-containing diets significantly depressed FC and EEF but had no effect on all other parameters. Dietary EA, independent of dietary type and level of CGBI significantly depressed EEF but had no effect on all other parameters. No significant CGB by EA interactions were observed for all criteria measured. Generally; from an economic point of view, it may be concluded thai, rice bran, wheat bran or their combination can be incorporated into broilers' diets at an inclusion rate of 15°/e, with no detrimental effects on the performance of chicks for growth and feed conversion. Also , under the conditions of the present study, the dietary supplementation with the exogenous enzyme mixture was an undue extravagance and failed to elicit any effect on either performance of chicks or nutrients digestibility. Thus it can be recommended that, such enzyme preparations should be subjected to an in vitro laboratory test to make sure of its activity; prior to using it in the application field as a feed supplement.