An experiment of 35d duration was conducted to evaluate the effects of dietary supplementation (1g/kg) of yeast (Saccharomyces cerevisiae), yeast protein concentrate (YPC) and YPC-pellets on production performance of heat stressed broilers (Cobb 400, n = 128). Ambient temperature was constantly maintained at 32-34°C. At 21d, all the birds were orally challenged with Salmonella enteritidis (10×10⁷ colony forming units/ml) and the numbers of total Salmonella were enumerated in excreta at periodic intervals. Dietary supplementation of YPC and YPC pellets improved body weight at 21 and 35d of age (P=0.001) and FCR in 35d (P=0.001). Salmonella numbers decreased in the pooled digesta and excreta due to dietary supplementation of yeast and YPC additives (P<0.05) particularly in the YPC-pellets group. E. coli numbers in digesta also decreased in the treated groups (P=0.011). Humoral immune response against Newcastle disease was improved by dietary supplementation of YPC and YPC-pellets at 14d (P=0.028), 28d (P=0.001) and 35d (P=0.001). The effect of dietary yeast supplementation on humoral immune response was evidenced on 35d of age. Severity of panting was comparatively less when the YPC additives were supplemented in diet and this was corroborated by a lower serum CO₂ concentration (P=0.019) in the YPC and YPC-pellets groups at 21d of age as compared with the control. Supplementation of YPC and YPC-pellets increased serum T3 level (P=0.001) at 21d and decreased serum cortisol at 21 and 35d vis-à-vis the control group (P=0.001). Yeast supplementation was also as effective as the YPC additives in reducing serum cortisol level. Villus height increased by supplementation of YPC-pellets as compared with the other dietary groups (P=0.008) although the implication of this increment was not discernible in terms of production performances. Although, converting the yeast protein concentrates into YPC-pellets did not offer any substantial advantage in terms of enhancing production performance of the heat stressed broilers, it may be concluded that YPC additives may effectively sustain the production performance in heat stressed broilers than the conventional yeasts probably by modulation of the levels of circulatory thyroid hormones and cortisol.