An experiment was conducted to evaluate the effects of microbial phytase and dietary Calcium (Ca) and phosphorus levels on productive traits, tibia ash percentage, apparent retention of Ca, tP, N and also on number of blood factors in male broiler chickens. The experiment was a 2×3 factorial with a completely randomized design. Each treatment consisted of 5 replicates of 12 male broilers, for a total of 360 chicks. Broiler chickens were fed two dietary levels of Ca and nPP (NRC, 1994 recommended levels and 80% of its recommendation levels) and three levels of phytase (0, 300 and 600 FTU kg-1 of diet) from 7-42 days of age. Experimental diets were formulated to be similar nutrients except for Ca and tP. During the experiment Body Weight (BW), Average Daily Gain (ADG), Average Daily Feed Intake (ADFI) and Feed Conversion Ratio (FCR) were measured weekly. Tibia ash percentage and blood serum Ca, P and Alkaline Phosphatase (ALP) were measured at 28 days of age. Nutrient apparent retention was measured at 42 day. Phytase addition increased (p<0.05) BW (21 and 42 day) and ADG during 7-21 and 7-42 days of age. FCR also was improved (p<0.05) during 7-21 and 7-42 day by phytase addition. Neither phytase addition nor Ca and nPP levels had significant effects (p>0.05) on ADFI during the periods of the experiment. Phytase addition decreased (p<0.05) serum ALP linearly and also decreased feed cost/kg BW yield. Low dietary Ca and nPP levels improved BW (21 day), ADG and FCR during 7-21 day (p<0.05). Phytase addition to low Ca and nPP diets was more efficient (p<0.05) than normal Ca and nPP diets on BW, ADG, ADFI, FCR, serum ALP concentration and Ca, tP and N retention. Phytase supplementation increased (p<0.05) serum P concentration, tibia ash percentage and Ca, tP and N retention. Low dietary Ca and nPP levels, increased (p<0.05) serum ALP and Ca, tP and N retention and decreased (p<0.05) serum P concentration. These data indicate that phytase improves BW, ADG, FCR and decreases serum ALP and feed cost/kg BW yield of broiler chickens. Low dietary Ca and nPP levels, regardless to phytase levels, improve BW, ADG and FCR. Phytase addition to low Ca and nPP diets is more efficient than normal Ca and nPP diets on performance and Ca, tP and N retention.

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