Previous research indicates that red cell hemolysis occurs following running. However it is not clear whether this is due to the specific mechanical stress of the actual footstrike process or to metabolic and/or other mechanical stresses incurred during the general circulatory route. To investigate the significance of the footstrike we attempted to control for metabolic and general circulatory stress to the red cell by having ten male triathletes each complete two separate 1 hour sessions of running and cycling at equivalent oxygen uptake (75% VO2 max), performed in random order 1 week apart. Plasma free hemoglobin concentration (PFrHg, immediately post-tests) and serum haptoglobin (Hp, at 1, 6 and 24hr post-tests) were measured as indicators of hemolysis. Met-hemoglobin (MHg) was measured from the immediately post-test samples as an indicator of red cell oxidation. PFrHg increased after both running (p<.01) and cycling (p<.01), but the increase was four-fold greater in running (p<.01). This was reflected by a significant fall in Hp 1 hour after running but no significant change occurred following cycling at any sample point. MHg increased two-fold after both running and cycling (p<.01) with no significant difference between modes of exercise. These data indicate that whilst stresses associated with the general circulatory demands of one hour of exercise at 75% VO2max may result in some exercise induced hemolysis, footstrike is a significantly more potent contributor.