The purpose of the present study was to evaluate the effect of acute bicycle exercise at different exercise intensities on the immune system. Six healthy volunteers exercised on a bicycle ergometer for 1 h at 25%, 50% and 75% of VO2max with an interval of 2 to 3 weeks. Blood samples were collected in the basal state, at the end of exercise and 2 h later. The absolute concentrations of all lymphocyte subsets increased during and fell after exercise at 50% and 75% of VO2max, but did not change significantly at 25% of VO2max. However, at all exercise levels, the percentage of CD3+ blood mononuclear cells decreased due to a decline in the fraction of CD4+ cells. This decline was most pronounced at 75% of VO2max. The fraction of NK cells expressing either the CD16 or the CD56 marker increased during exercise and declined to prevalues 2 h later, however the changes were most pronounced at 75% of VO2max. The natural killer (NK) cell and lymphokine activated killer (LAK) cell activities (lysis per fixed number of mononuclear cells) were increased during all exercise intensities, but were only suppressed below basal levels after exercise at 75% of VO2max. Indomethacin in vitro abolished the post-exercise suppression of NK cell activity and the proportion of CD14+ monocytes increased 2 h after exercise only at 75% of VO2max. These findings indicate that after exercise NK cell function is inhibited by prostaglandins released by monocytes. During exercise at 50% and 75% of VO2max the proliferative response of blood mononuclear cells (BMNC) following stimulation with phytohaemagglutinin A (PHA) decreased, whereas that following stimulation with interleukin-2 (IL-2) was enhanced. The IL-2 production by BMNC in vitro was markedly decreased during and after exercise at 75% of VO2max and this inhibition could be abolished by indomethacin in vitro. In conclusion, the response of the immune system to exercise depends on exercise intensity. In essence, the response is enhanced during exercise, however, after heavy exercise it is suppressed due to an increased level of prostaglandins produced by the elevated number of monocytes.