Five male endurance-trained runners completed an interval running session of 15 x 1-min intervals at 95% VO(2)max. Venous blood samples were collected pre-exercise and then immediately, 30- and 60-minutes post-exercise. The response of cultures of total lymphocytes to mitogen (phytohaemagglutinin) were significantly reduced immediately after exercise, but returned to resting levels by 30-min of recovery. Conversely, the mitogen response of cultures of pure T-lymphocytes (CD4(+) and CD8(+) cells), separated using a magnetic separation technique, showed no significant change during the exercise and recovery periods. These data showed directly that there was no apparent change in the functional capability of T-lymphocytes following an intensive interval training session. Furthermore, there were significant changes in the composition of the total lymphocyte cultures immediately post-exercise; increased numbers of natural killer (NK) cells (CD56(+)) and T-suppressor cells (CD8(+)) and decreased numbers of T-helper cells (CD4(+)). There were also significant correlations between total mitogen response and the composition of the cultured lymphocytes. These data indicated that the large increases in MK cells, relative to T-cells, following intensive exercise, were the most likely cause of the reduced mitogen response of total lymphocyte cultures.