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The purpose of this study was to examine the relationship between active compared to inactive lifestyles and immunocompetence in men. Subjects, all male volunteers, regularly exercising moderately were separated into three age groups: young (20-39 years), middle-aged (40-59 years) and elderly (more than 60 years). Age-matched sedentary male subjects served as controls in each group. Immunological assessments were, total leucocyte count, lymphocyte subpopulation counts, natural killer cell activity and neutrophilic phagocytosis. Total leucocyte and T-cell (CD3+) counts were not significantly different among the groups. Among T-cell subsets, there was a slight increase in helper T-cell (CD3+CD4+) and a decrease in cytotoxic/suppressor T-cell (CD3+CD8+) concentrations in the older sedentary subjects, resulting in an age-associated significant increase in the CD4:CD8 ratio among those control groups. However, among the exerciser groups, no such increase and decrease in the T-cell subpopulations or an age-related increase of the CD4:CD8 ratio were observed. Considering the components of innate immunity, the concentration of NK-cells (CD16+CD56+) significantly increased in the elderly exercisers, compared to that of the age-matched control subjects, or of the young group. The phagocytotic activity of neutrophils showed an age-associated decline, but of lesser degree in the elderly exercisers than in the elderly controls. Taken together, these results suggest that habitual and moderate training in later life is associated with a lesser age-related decline in certain aspects of circulating T-cell function and innate immunity.