The experiment was conducted to evaluate CM (Cell Mass from Lysine Fermentation), which is used to produce synthetic lysine in industry, as an alternative protein source in broiler diets. Three different production conditions were employed to produce CMs (CM I, II, III, Treatments were control, CM I-1 (1% of CM in the diet), CM I-3 (3% of CM in the diet), CM I-5 (5% of CM in the diet), CM II (3% ... [Show full abstract] of CM in the diet), and CM III (3% of CM in the diet). It was found that CM products were all high in crude protein content and especially high in lysine and methionine contents, while very low in minerals. For the starter period, all CM group showed better weight gain, chicks fed CM I-1 diets were especially high in weight gain (p<0.05). CM groups consumed 14.4 to 18.0% more feed than chicks fed control diets (p<0.05). The best FCR was found in CM I-1 groups (p<0.05), but as CM level was increased, FCR was also increased. For the finisher period, weight gain was similar through all treatments. Through whole experimental period, weight gain and feed intake were higher in all CM groups than control group (p< 0.05), however, as CM level was increased, FCR was also increased. Generally chicks fed CM diets showed higher utilizabilities of gross energy, dry matter, crude protein and crude fat. The best nutrients utilizability was obtained in CM I-1 group, and the worst was found in the control group. During the finisher period, the utilizabilities of crude protein, crude ash and phosphorus were not affected by the dietary treatments. Amino acids utilizability was not significantly affected by the treatments except CM I-5 group. In all amino acids tested, chicks did not show the big difference in utilizabilities. Only in the CM I-5 group, amino acids utilizability was significantly lower than control group. However, among CMI groups, the mean value of the amino acids utilizability was decreased as the level of CM inclusion in the diet was increased. During the finisher period, similar trend was found in amino acids utilizability.