The behaviour of the intrinsic fibrinolytic system was studied in healthy donors using the blood activator inventory test of Kluft. Moreover, the influence of several pre-analytical factors was examined. The activity of the intrinsic system was higher in the afternoon than in the morning. Medium physical stress caused an increase in activity which was not due to the influence of extrinsic activators. By inhalation of cigarette smoke the activity was decreased. In healthy volunteers the values of the test showed rightward shifted frequency distribution. Females between the age of 20 and 39 yr had significantly higher intrinsic fibrinolytic activity than males of the same age group. In both sexes the activity tended to decreasing values with increasing age.