The effect of heavy short-term physical exercise on the levels of complement receptor type one (CR1, CD35) on erythrocytes, the concentrations of circulating immune complexes (IC), and the complement C3 split products C3c and C3d were examined in young healthy males. Fourteen untrained volunteers underwent a 60-min bicycle exercise test at 75% of maximal oxygen uptake (VO2max). Six of the volunteers were exercised twice with an interval of at least one month. Before the second bicycle test they received oral indomethacin. With an interval of at least 1 week, 6 also went through a 60-min back-muscle exercise at up to 30% of VO2max. Blood samples were collected before and during the last few minutes of exercise as well as 2 h and 24 h afterwards. The same parameters were examined once in 29 highly trained racing cyclists. There were no consistent or significant exercise-induced changes in the levels of erythrocyte CR1, circulating IC, C3c nor C3d as measured by an enzyme-linked immunosorbent assay, polyethylene glycol precipitation complement consumption method, and by intermediate gel rocket immunoelectrophoresis, respectively. Neither did these parameters differ from controls in the highly trained group. The results indicate that CR1 on erythrocytes, circulating immune complexes and complement cleavage products C3c and C3d in healthy subjects remain unaffected by short-term heavy physical activity and training.