To determine if the complement system is activated following strenuous eccentrically-biased exercise. Secondly, to determine if complement activation is attenuated (repeated bout effect) following a second bout of the same exercise. Basic procedures. Healthy, active but untrained males performed 2 × 60 min bouts of downhill running, 14 days apart. Samples were taken pre, immediately post (IP), then every hour for twelve hours, and at 24, 48, 72, 96, 120 and 144 h post exercise.