Using diagnostics for the determination of clotting factors and fibrinolytic parameters in human plasma, samples from rat, guinea pig, rabbit, cat, dog, sheep, cattle, horse, pig, and monkey were analysed. The human system was employed even for standard curves and controls. Results obtained in this way are relative values in relation to pooled fresh human plasma of healthy donors which is defined to contain 100% of the norm or 1 unit of each factor per 1 ml. Under these conditions, marked differences between the human clotting system and those of different animal species appear. Thus, rabbits have an about 50 times higher activity of factor V than humans and guinea pigs only 6% of the factor VII activity of human plasma. Plasma levels of fibrinogen, alpha 2-antiplasmin and antithrombin III in plasma samples of the investigated animal species are in the range of human plasma. Differences in the plasma level of single factors as compared to human plasma are reflected by clotting times of the screening tests. For the determination of plasminogen, streptokinase was used as activator of the fibrinolytic system. Hence, the results obtained by this method merely reflect the activatability of the animal plasminogen in comparison to the human system, however, do not allow statements concerning the real plasminogen content of the plasma sample from the animal species. For pharmacological investigations a proper selection of the animal species is important to prevent wrong conclusions.