The effect of eccentric one-legged exercise on natural killer (NK) cell activity was studied in eight healthy males. To distinguish between local and systemic effects, blood samples were collected from veins in the exercising leg and resting arm. However, the results did not significantly differ between the leg and arm. To eliminate diurnal variations, the results were compared with a control group that did not exercise but had blood samples collected at the same time points. In the exercising group, plasma creatine kinase increased progressively during and up to 4 days after exercise. The percentage of CD16+ NK cells increased during exercise, which was paralleled by an increase in the NK cell activity per fixed number of blood mononuclear cells. The NK cell activity on a per NK cell basis did not change. The percentage of CD3+, CD4+, CD8+, CD19+, and CD14+ cells did not change significantly during exercise. The present study thus showed that eccentric exercise with a relatively small muscle mass (1 quadriceps femoris muscle) causes systemic effects on NK cells. It is suggested that the increase in plasma epinephrine during eccentric exercise is responsible for the observed increase in the percentage of CD16+ cells.