Abstract Background Alternatives to antibiotics have been suggested by banning their use in the poultry industry. Garlic and mushroom are two important phytobiotic compounds in poultry nutrition. Objectives This study was conducted to evaluate the effects of supplementing diets with garlic and mushroom powder (MP) on growth performance, humoural and cellular immunity, and white blood cell counts of broiler chickens. Methods Five hundred and seventy-six 1-day-old male broiler chickens (Ross 308) were assigned to 8 treatments with 6 replications (12 birds per replication) based on a completely randomized design in a factorial arrangement of 4 × 2 with 4 levels of garlic powder (GP; 0.00%, 0.50%, 1.00%, and 1.50%) and two levels of MP (0.00% and 1.00%). Results No significant effects of GP and MP on the growth performance and cutaneous basophil hypersensitivity were observed (p > 0.05). According to the regression equation, with increasing levels of GP in the diets, the relative weight of the bursa of Fabricius and thymus increased (p < 0.05). The effect of increasing levels of GP on the Newcastle disease virus (NDV) titre was greater in the absence of MP (p < 0.05). With increasing levels of GP in the diets, the percentages of lymphocytes and heterophils to lymphocytes ratio increased and reduced, respectively (p < 0.05). Conclusions This experiment has revealed that increasing the level of GP improved the immune response of broilers without affecting performance. The effect of increasing the level of GP on the NDV titre was more significant in the absence of MP.

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