A 42-day study was done to explore the application of supplemental amino acids (AA) in reduced crude protein (CP) diets with SBM, CM or DDGS as the main protein feedstuffs. Responses of interest were growth performance, carcass yield, whole-body composition, litter ammonia and litter N. A total of 810 Cobb 500 male broilers were randomly allocated to 54 floor pens, with 15 birds per pen on d 0. All the birds received the same starter diets (22% CP, 3000 kcal/kg AME) during the first 10d. Thereafter, 9 experimental diets(D) were provided in both grower (d10-28, 18.5% vs 14% CP for adequate and low CP, respectively) and finisher (d28-42, 17% vs 13% CP for adequate and low CP, respectively) phases -; D1 was adequate CP corn-SBM positive control diet. The CP of D2 and D3 (corn-SBM based) was decreased by 2 or 4.5%, respectively relative to D1. Diets-, D4, D5 and D6 had the same CP levels as D3 but with increasing DDGS inclusion levels (5%, 8.75%, 12.5%, respectively). Diets D7, D8 and D9 had the same CP level as D3 but with CM inclusion levels of 5%, 7.5%, 10% respectively. Diets 2 to 9 (the low-CP diets) had same level of standardized ileal digestible indispensable AA with supplemental indispensable AA added as required. Gly and Ser were added as sources of N. Bird weight and feed intake were measured on d 10, 28 and 42. Whole body composition was determined using dual energy x-ray absorptiometry on d 21 and 42, whereas litter N, ammonia and carcass yield were determined on d 42. Serial reduction in dietary CP with SBM linearly reduced (P < 0.05) weight gain, feed intake and gain to feed. Increasing levels of DDGS and CM, at constant CP level, also linearly (P < 0.05) reduced weight gain, feed intake and gain to feed ratio. Breast yield linearly reduced (P < 0.05) with serial reduction in CP with SBM, but no significant differences were observed between SBM, DDGS or CM diets at the same CP and AA levels. Litter N and ammonia increased (P < 0.05) with increasing levels of CP in SBM diets.