Heat stress (HS), one of the major problems of tropical and subtropical countries, adversely affects the production performance of poultry. Keeping this in view, the present study was designed to investigate some of the biological markers of HS in broilers as modulated by dietary supplementation of mannan-oligosaccharide (MOS) and a Lactobacillus-based probiotic (LBP), either alone or in combination. Two hundred fifty 1-d-old-chicks were randomly divided into 5 groups. From d 22, the birds were either kept at the thermoneutral zone (TN) or exposed to HS to the conclusion of study, d 42. Birds were fed either a corn-based basal diet (TN and HS groups) or the same diet supplemented with 0.5% MOS (HS-MOS group), 0.1% LBP (HS-LBP group), or their combination. Birds were immunized against Newcastle disease virus on d 4 (intraocular; live attenuated) and d 20 (drinking water; live attenuated) and infectious bursal disease virus on d 8 (intraocular; live intermediate strain) and d 24 (drinking water; live attenuated). Birds were killed on d 42 to collect serum for determination of cortisol, thyroid hormones, cholesterol, C-reactive protein, and postvaccinal antibody titers. Results revealed that dietary supplementations decreased (P < 0.05) the serum cortisol and cholesterol concentrations and increased (P < 0.05) thyroxine concentration compared with the HS group without affecting triiodothyronine concentration. The percentage of the C-reactive protein-positive birds was higher (P < 0.05) in the HS group compared with the TN group. Dietary supplementations improved humoral immunity against Newcastle disease virus and infectious bursal disease virus during HS. In conclusion, dietary supplementation of either MOS or LBP alone or in combination can reduce some of the detrimental effects of HS in broilers.