Abstract: Hematological responses in chicken with different HSP 70 genotypes to acute heat stress were studied using 28 kampong chickens, 24 Arabic chickens and 4 commercial chickens. The experimental chickens were selected randomly from a group of chickens with HSP 70 genotypes identified and were exposed to ambient temperature (40°C) for 0.5, 1.0 and 1.5 h. Results showed that erythrocyte, hematocrit and leukocyte in all chicken lines decreased in response to acute heat stress with the highest decrease in commercial chickens, followed by Arabic and kampong chickens. Regardless of acute heat stress exposure, there was no significant difference in erythrocyte, hemoglobin, leukocyte, heterophil, eosinophil, basophil, lymphocyte, monocyte and heterophil/lymphocyte ratio in all chicken lines studied. Arabic and commercial chickens had lower hematocrit as compared to kampong chickens. However, acute heat stress increased the percentage of heterophil, basophil, lymphocyte and heterophil/lymphocyte ratio without affecting eosinophil and monocyte. It was found that there was no interaction between lines of chicken and acute heat-stress exposure on the hematological parameters measured. The lowest percentage of lymphocytes was found in chickens with DD HSP 70 genotype while the highest was found in chickens with AD genotype. The results indicated that there was a relationship of heat resistance or tolerance to lymphocyte expression. Chickens that were the most tolerant to acute heat stress had the highest lymphocyte percentage (AD genotype) whereas those that were the least tolerant had the lowest lymphocyte percentage (DD genotype).