Recently, the numbers of city marathon runners and joggers have increased. They believe that exercise can enhance their fitness level and health maintenance. However, many researchers have suggested that exhausting exercise may increase the risk of infection and free radical generation. In this study, eight male and ten female city marathon runners who participated in the Akita Nairiku Resort Cup 100-km Challenge Marathon Race were studied using luminol-dependent chemiluminescence (LDCL) to examine the ability of their neutrophils to produce reactive oxygen species. Furthermore, serum opsonic activity using LDCL, serum concentrations of the third and fourth components of the complement (C3, C4), the total number of leukocytes, and proportion of leukocytes were also determined. Evaluation of LDCL was undertaken using the maximum light emission (peak height) and the time to reach the maximum light emission (peak time) in the response curve of LDCL. Following the race, the peak time was prolonged by 11% in males and 15% in females, and the peak height increased by 68% in males and 48% in females. These results suggest that exhausting exercise stimulates neutrophils to produce reactive oxygen species. The total number of leukocytes increased about twofold, whereas the number of neutrophils increased about threefold after the race as compared to before. The rate of the increase in leukocytes was closely associated with that of neutrophils (r = 0.96, P < 0.01). Therefore, the leukocytosis observed in this study was dependent on neutrophilia. Serum opsonic activity and serum concentrations of C3 and C4 were not changed following the race. The increase in the ability of neutrophils to produce reactive oxygen species and neutrophilia after the race, suggests that a large quantity of reactive oxygen species may be produced in vivo following exhausting exercise.