This study was designed to examine the impact of exhaustive endurance exercise on a number of immune parameters of physically fit male subjects (VO2max 66.5 ± 5.3 ml/min/kg) who performed treadmill exercise at 65 % of their /O2max for 120 min. Serial blood samples were taken before, during and after exercise and changes in leukocyte and lymphocyte subset concentrations; immunoglobulin production in vitro; and natural killer (NK) cell response were measured. The exercise regimen was found to induce the well-known phenomenon of leukocytosis which consisted primarily of a granulocytosis and lymphocytosis. Among the lymphocyte subsets, peripheral pan T cells (CD3+) as well as helper (CD4+) and suppressor (CD8+) T cells were found to be elevated. A relatively smaller increase in CD4+ than CD8+ cells resulted in depressed CD4/CD8 ratios throughout the exercise period. After exercise, T cells declined progressively and, 2 h post-exercise, were less than 60 % of their pre-exercise level. In contrast, the CD4/CD8 ratio demonstrated a progressive increase, thus representing a reversal in the pattern observed during exercise and a trend towards an elevated ratio during recovery. B cells (CD19+) were relatively unaffected by exercise, although IgM production by pokeweed mitogen-stimulated lymphocytes obtained from blood samples after 120 min of exercise was significantly depressed. NK cells were affected dramatically by exercise. Both CD16+ cell numbers and NK cytotoxicity were increased during exercise, followed by a persistent depression in the post-exercise period. The strenuous exercise induced a profound effect on NK cells as evidenced by a 40 % depression of the NK cell count for as long as 7 days after the cessation of exercise. Our results provide direct kinetic evidence demonstrating that exhaustive exertion alters both lymphocyte distribution pattern and effector function, suggestive of possible exercise-induced immune compromise, particularly in the post-exercise recovery period.