Continuous-flow electrophoresis was used to isolate purified plasminogens from the serums of 10 species. The ratios of esterolytic to caseinolytic activity in the purified plasminogens with urokinase as the activator for the following animals were: rat 9.8, cow 6.6, pig 5.7, cat 3.5, sheep 3.4, mouse 3.3, rabbit 2.7, man 2.2, monkey 2.0, and dog 1.4. Plasminogen activation with streptokinase divides the species into three groups: (a) activated with small amounts of streptokinase (man, cat, monkey), (b) activated with large amounts of streptokinase (dog, rabbit), and (c) not activated (cow, sheep, pig, mouse, and rat). Streptokinase – human globulin mixture and urokinase activated all plasminogens equally well When the euglobulin fractions of the serums of nine of the ten species were subjected to starch gel electrophoresis at pH 2.5, groups a and b gave two major plasminogen bands, whereas c gave only one.