Changes were explored in the behavior of circulating monocytes and their potential association with the activation of the coagulation system as assessed following strenuous exercise. Twelve men and nine women from the Norwegian national cross country skiing team and 19 men and six women from a level just below that of the national team were studied before and after ski race competition. Mononuclear cells were isolated after incubation of heparinized blood with lipopolysaccharides (LPS; 3 ng·ml−1) for 2 h. After a 50 km race for men, the specific thromboplastin activity of the stimulated monocytes rose from 3.5 x 10−3/106cells to 21.4 x 10−3/106cells. This probably reflects the mobilization of a new population of monocytes that are more sensitive to such stimuli. Resting top-athlete skiers had monocytes which were significantly less responsive to the LPS stimulus compared to nontrained people. There was an inverse correlation of plasma factor VII and the monocyte responsiveness toin vitrostimulation (r = 0.814;P< 0.002) from blood drawn after a race. Furthermore, factor VII was significantly reduced after a 50 km race, and a modest decline in the fibrinogen level was also observed (P< 0.05). It is concluded that endurance ski racing causes white cell mobilization and more active white cells that may induce activation of the coagulation system and account for the involvement of factor VII and fibrinogen.