This study examined the effect of supplementary selenium on leukocytes and heat shock protein (HSP) 70 expression in serum during half-body immersion. The subjects were male college tennis athletes. All subjects participated in two repeated experiments with a 1 week interval. During the 30 min intermittent half-body immersion, subjects were given 500 mL of water with or without selenium (100 ${mu}g$). Blood samples were taken from the antecubital vein, and differential counts were made. Serum HSP70 protein was analyzed using a commercial ELISA kit. After half-body immersion, leukocytes and lymphocytes increased significantly but neutrophils decreased significantly in both trials (with or without selenium). Selenium supplementation, compared with placebo, decreased levels of leukocytes, neutrophils, and monocytes, but not lymphocytes, to the resting level or below 60 min after immersion. Only lymphocytes continued to increase in both trials during the recovery period. Serum HSP70 protein level did not change after immersion, but it decreased 60 min after immersion with the administration of selenium. In conclusion, supplementary selenium reduced the systemic immune response and serum HSP70 protein accumulation after half-body immersion.