Exercise has been recognized to provoke upregulation of antibodies. However, the mechanism has not been explained. We examined the effects of voluntary wheel-running exercise on the number of cells which produce tetanus toxoid (TT)-specific IgG, as well as serum level and clearance of administered 125I-labeled mouse IgG in the blood. Male C57BL/6N mice were randomly divided into a voluntary wheel-running exercise group and a sedentary group. Mice were intraperitoneally immunized with 0.375 microg/kg of TT to induce primary and secondary anti-TT antibody responses. ELISPOT assays that identified TT-specific antibody production were performed on day 0 (Baseline, n = 8) and 22 (EX: n = 8, Non-EX: n = 8) after initial immunization (primary response) and on day 32 (EX: n = 8, Non-EX: n = 7) and 43 (EX: n = 7, Non-EX: n = 7). To explain why serum TT-specific IgG was elevated in the exercise group, we conducted an 125I-labeled mouse IgG clearance test on day 32. ELISPOT counts of secondary responses to TT immunization were significantly higher in the running group than in the sedentary group (P<0.05). The serum anti-TT specific IgG concentration was also significantly higher in the running group (P<0.05) than in the sedentary on day 32. The values of both groups were relatively lower on day 43. The (125)I-labeled mouse IgG was more rapidly cleared in the non-exercised than in the exercised group (P<0.05). These results show that voluntary wheel running upregulates the TT-specific humoral immune response. These reactions may be partly explained by the accelerated induction of TT-specific IgG-producing cells and prolonged serum IgG half-life with voluntary exercise.