The purpose of this study was to evaluate the effect of esterified glucomannan (E-GM) on performance, immunity, blood haematological and serum biochemical parameters in broilers exposed to diets naturally contaminated with mycotoxins. A total of 630 one-day-old male broiler chicks (Ross 308) were randomly assigned to 9 treatments and 5 replicates of 14 chicks based on factorial (3 × 3) arrangement in completely randomized design. The dietary treatments included 3 levels of substituting naturally contaminated corn (0%, 50% and 100%), three levels of E-GM (0%, 0.05% and 0.1%) and their interaction. Body weight gains (BWG), feed intake (FI) and feed conversion ratio (FCR) were evaluated from 7 to 49 days of age. Haematology, serum biochemical and enzyme activities were assessed. Antibody titre against Newcastle disease virus and infectious bursal disease was measured to evaluate the humoral immunity. In comparison to diets with no contamination, 50% and 100% naturally contaminated corn significantly decreased FI, BWG and FCR (P < .05). Supplementing 0.05% and 0.1% E-GM considerably improved the decreased BWG and FI (P < .05). However, only 0.1% binder ameliorated the negative impact of mycotoxins on FCR (P < .05). Replacement of contaminated corn remarkably diminished humoral immunity of chickens and increased liver enzyme activities which ameliorated by supplementing 0.05% and 0.1% of binder inclusion (P < .05). Results indicated that supplementing E-GM particularly at 0.1% level efficiently reversed the adverse effects of mycotoxins on broiler chickens. ARTICLE HISTORY