The present study was designed and conducted to assess the physiological effects of aflatoxin and microbial contamination induced stresses on hepatic antioxidant and immunological gene expression and biochemical blood parameters in broiler chicks. In the first trial, a total of 528 seven-day-old Ross broiler chicks were randomly assigned into 11 experimental treatments with 4 replicates of 12 birds each. Experimental treatments consisted of a control group (unchallenged group), 2 aflatoxin levels (0.5 and 2 ppm) and 5 situations in terms of additives of mycotoxins detoxifiers in each aflatoxin level (no additive, 2 mannanoligosaccharides levels 0.1 and 0.2% and 2 silymarin levels 400 and 800 mg/kg). Birds were challenged with a mixture of aflatoxins during 7 to 28 d of age. In the second trial to evaluate microbial stress, 672 one-day-old chicks were used. Escherichia coli O78:K80 bacteria at 10 9 cfu/ml were orally given to birds from 7 to 28 d of age. Experimental treatments consisted of two microbial conditions (unchallenged group and Escherichia coli challenge) and seven dietary additive supplementations (no additive, 500 and 1000 IU/kg lantibiotic, 400 and 800 mg/kg silymarin, a combination of 1000 IU/kg lantibiotic and 800 mg/kg silymarin and also baitracin) in a 2×7 factorial arrangement with 4 replicates of 12 birds each. Results of the first trial showed that aflatoxin challenge led to the increases in cholesterol (P 0.001), triglyceride (P 0.0001), glucose (P 0.001), urea (P 0.0001), albumin (P 0.0001) and malondialdehyde (P 0.0001) concentrations, the noticeable increases in alanine aminotransferase, aspartate aminotransferase and alkaline phosphatase activities (P 0.0001) and also the declines in total protein (P 0.0001) and glubolin (P 0.0001) concentrations in serum of broiler chicks. Additionally, aflatoxin contamination caused a reduction in antibody titer against Gumboro disease virus (P 0.01), the hepatic downregulations in interleukin 2 (P 0.01), glutathione peroxidase and superoxide dismutase (P 0.0001) gene expressions and also the hepatic upregulations in interleukin 6 and C reactive protein gene expressions (P 0.001). Also, aflatoxin exposure resulted in the increased ileal Escherichia coli , Salmonella , Klebsiella and total negative bacteria (P 0.0001), the increased crypt depth (P 0.0001), goblet cell counts (P 0.0001) and lymphoid follicular diameters (P 0.01) in jejunum as well as the decreased (P 0.0001) villi height, villi height to crypt depth ratio, villi absorptive area and apparent villi absorptive area in broilers. Furthermore, exposure to aflatoxin caused the decreases in feed intake (P 0.001), daily weight gain (P 0.001) and a rise in feed conversion ratio (P 0.0001) in chicks. Dietary supplementations of either mannanoligosaccharides or silymarin had significant effects regarding to total antioxidant capacity (P 0.001), serum enzyme activities (P 0.05), gene expressions of interleukin 2 (P 0.05) and glutathione peroxidase (P 0.01), ileal microbial enumerations (P 0.05), villi absorptive area (P 0.05) and feed conversion ratio (P 0.05) in broilers subjected to aflatoxin. Results of the second experiment showed that Escherichia coli challenge O78:K80 resulted in the increases in glucose, urea, creatinine and malondialdehyde concentrations (P 0.0001), the increases in serum alanine aminotransferase, aspartate aminotransferase and alkaline phosphatase activities (P 0.0001), a noticeable elevation in heterophil to lymphocyte ratio (P 0.0001), the upregulations in interleukin 6 and C reactive protein gene expressions (P 0.0001), the significant increases in ileal Escherichia coli , Salmonella , Klebsiella and total negative bacteria (P 0.0001), a rise in feed conversion ratio (P 0.0001) as well as the marked decreases in serum total protein and glubolin contents (P 0.0001), the reductions in antibody titers against Newcastle disease virus (P 0.001), bronchitis (P 0.0001) and Gumboro disease virus (P 0.01), a decline in lymphocytes percentage (P 0.0001), the hepatic dowuregulations of interleukin 2 (P 0.0001) and superoxide dismutase (P 0.001) gene expressions, a reduction in total antioxidant capacity (P 0.0001) and the depression in villi absorptive area (P 0.001) in broilers. There were significant interactions in terms of serum enzyme activities (P 0.0001), humoral immunity (P 0.05), cellular immunity (P 0.0001), hepatic immunological (P 0.0001) and antioxidant (P 0.01) gene expressions, serum antioxidant indices (P 0.0001), ileal microbial counts (P 0.05), daily weight gain (P 0.05) and feed conversion ratio (P 0.0001) in broilers exposed to Escherichia coli . .