Five hundred day-old male Ross broiler chicks were divided into 50 groups of 10 birds each and randomly assigned to ten dietary treatments with 5 replicates each. Two corn-soybean meal-based diets were first provided to meet 100% and 95% of recommended digestible amino acids (RDAA) for starter (1-14 d of age), grower (15-35 d of age) and/or finisher (36-49 d of age) periods. Each starter diet were subdivided into 5 parts and supplemented with 0 and 2 g kg-1 Fermacto and 0.2, 0.4 and 0.8 g kg-1 Cumin Essential Oil (CEO). The grower diets contained half of the same supplementation of the starter diets and each of the un-supplemented finisher diet fed to the corresponding birds. The performance parameters were determined during all periods and carcass yields and relative organ weights measured on d 28 and 49. Gastrointestinal transit time (GTT) and fat digestibility were determined on d 16. The blood metabolites concentration, cell differentiation and total anti-SRBC, IgG and IgM titres measured on d 26 in birds fed diet containing 100% RDAA level. Cumin essential oil and Fermacto did not have a significant effect (P>0.05) on performance parameters in the starter and finisher periods but higher BWG was observed in birds fed diet with the lowest level of CEO supplementation in the grower period. Birds fed diet with 100% RDAA and Fermacto had higher feed: gain ratio in the grower period when compared to those fed diet of similar AA and 0.2 g kg-1 of CEO. A 5% decrease in the RDAA had no adverse effect (P>0.05) on the overall feed consumption and BWG, but feed: gain ratio was significantly (P>0.05) increased. Carcass yields and cuts of broiler chick were not significantly influenced (P>0.05) by CEO, Fermacto and/or DAA levels. There was not a significant difference (P>0.05) in total anti SRBC, IgG and IgM titres, GTT and fat digestibility in bird fed 100% RDAA diets supplemented with either CEO or Fermacto. The inclusion of Fermacto and/or various levels of CEO in diets did not have an influence (P>0.05) on serum metabolite concentrations (mg/dl) at d 28, although triglyceride and VLDL concentrations was lower (P<0.05) in chicks fed starter diet contained 0.4 g kg-1 CEO.

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