The study evaluated supplementation of broiler diets with a protease enzyme. Six hundred Ross 308 female broiler chicks were acquired at one day of age and randomly distributed to three treatments with five replicates of 40 chicks each. The experimental treatements consisted of i) a positive control diet with normal levels of crude protein, digestible lysine and digestible sulphur containing amino acids (PC); ii) a negative diet with reduced levels of crude protein, digestible lysine and digestible sulphur containg amino acids (NC); and iii) the NC diet augmented with Ronozyme ProAct protease at 200 mg/kg (NC+ENZY). Compared to the other diets, feeding the PC produced the highest final bodyweight (BVV), bodyweight gain (BWG), and feed intake (FI) and best feed conversion ratio (FCR) over 0 - 42 days. The PC diet also produced the highest hot and cold carcass yields, ileal protein digestibility and improved the serum biochemistry parameters. Relative to the NC, the NC+ENZY diet increased BW, BWG and FI, improved FCR, increased ileal protein digestibility, hot and cold carcass yields, and the crude protein content of meat. However, it decreased serum triglyceride and very low density lipoprotein (VLDL) cholesterol levels and the crude fat content of meat of broilers. As a result, protease supplementation at the level of 200 mg/kg may not remove the detrimental effects that are caused by a 6% reduction in crude protein and digestible amino acids on performance, hot and cold carcass yields and ileal digestibility of crude protein of broilers.