Summary1. Caseinolytic and fibrinolytic systems for assay of plasmin in fibrinolytic agents are described which are based upon the determinations of AE/min during the linear portion of the casein-plasmin and fibrin-plasmin reaction curves respectively. A " caseinolytic-rate " unit and “fibrinolytic-rate " unit of ÄE/min × 103 during the linear portion of the respective curves are proposed.2. Data are presented indicating that a reliably linear relationship exists between plasmin concentration and these caseinolytic - and fibrinolytic-rate units.3. Data comparing results obtained with the proposed assay techniques and previously-used casein and fibrinolytic techniques are presented.4. Formulae by which caseinolytic-rate and fibrinolytic-rate units can be roughly converted into Remmert-Cohen type plasmin units are offered.5. The theoretical and practical problems which have influenced development of assays for fibrinolytic components are discussed.6. The advantages of the plasmin “rate unit” techniques vis a vis existing assays are delineated.7. The potential application of the techniques to measurements other than the plasmin content of fibrinolytic agents is discussed.