The present study was carried out to investigate the performance of broiler chicks fed isocaloric experimental diets (3170 k cal/kg) containing 18%- or 20%-plant protein, during the growing period (15 to 42 days of age) in absence or presence of exogenous commercial enzyme preparations (Phytase, Natuzyme, Sicozyme or Avizyme) at a level of 0.5 g/kg diet. Three hundred and sixty one-day old Cobb-500 broiler chicks were kept in battery brooders and fed a common starter diet (ME; 3000 k cal/kg and 21.57% CP) up to 14 days of age. Then, they were transferred wire-floored growing batteries, according to a factorial design (2×5) were distributed into ten equal groups of three replications each, and fed their respective experimental diets up to 42 days of age. All chicks had free access to feed and water and managed similarly. The criteria of response were live body weight, weight gain, feed intake, feed conversion, economic efficiency, carcass traits, nutrient digestibility and some blood parameters (plasma levels of glucose, total protein, total lipids and cholesterol as well as activity of alkaline phosphatase, alanine aminotransferase and aspartate aminotransferase in blood plasma of chicks). The obtained results could be summarized as follows: Regardless of the effect of dietary enzyme supplementation, broiler chicks fed the 20%CP-diets consumed significantly more feed and exhibited superior means of final live body weight, weight gain, feed conversion, nitrogen retention and CP digestibility as compared to those fed the 18%CP-diets. Apart from the effect of dietary protein level, adding either type of enzyme preparations to the diets produced significant positive effects on final live body weight of chicks, and equally exerted significant beneficial effect on digestibility of CP and EE, and nitrogen retention as compared to those fed the non-supplemented diets. Neither dietary protein level nor supplemental enzymes gave significant differences in economic efficiency, carcass traits and blood constituents of chicks. From the previous results, it could be concluded that in these plant-protein experimental diets the level of 20% CP was more suitable achieving satisfactory growth performance of broiler chicks during the grower period. In addition, dietary enzyme supplementation had some positive effect on chicks' growth performance. Natuzyme and Sicozyme brought about the best results with the present experimental diets.