This study examined the influence of a triathlon on the immune system and on serum amino acid concentrations. Eight male triathletes swam 2500 m, bicycled 81 km, and ran 19 km. The concentration of total serum amino acids decreased during the race, with the lowest values occurring 2 h postexercise. Similarly, serum glutamine concentration declined from 468 (SEM 24) (prerace) to 318 (SEM 20) mumol-1 (2 h postrace) and the natural killer (NK) and lymphokine activated killer (LAK) cell activities were suppressed 2 h postexercise (P < 0.05). Blood mononuclear cell proliferation decreased during exercise with the lowest value observed after running. The leucocyte concentration increased during and after exercise due to an increase in the concentration of neutrophils and monocytes. There was no significant change in lymphocyte concentration during or after the exercise. The plasma concentration of interleukin-6 did not change and the plasma concentration of interleukin-1 beta and tumor necrosis factor-alpha were below detection limits. The LAK cell cytotoxicity, but not NK cell activity or proliferative response, was significantly correlated with serum glutamine concentrations (r = 0.39, P < 0.01). This study confirms that prolonged endurance exercise results in changes in the cytotoxic function of the NK and LAK cells as well as the proliferative response. The time-course of changes in serum glutamine concentrations were best parallelled by changes in LAK cell activities.