.....We examined the effects of age and viral serology on the exercise responsiveness of γδ T-cells. Blood was collected from 17 younger (age: 23-35yrs) and 17 older (50-64yrs) healthy males matched for cytomegalovirus (CMV), Epstein-Barr virus, herpes simplex virus-1 and Parvovirus B19 serologic status before and after a single bout of cycling exercise. Older had lower numbers and proportions of γδ T-cells than younger, while CMV was associated with increased numbers and proportions of γδ T-cells in younger but not older. Exercise evoked a ∼2-fold increase in circulating γδ T-cell numbers. The magnitude of this response was 3-times greater in younger compared to older, and 1.6-times greater in younger CMV-infected compared to younger non CMV-infected. To conclude, γδ T-cell numbers and exercise responsiveness decreases with age and may contribute to impaired immunosurveillance after acute acute physical stress.