Abstract: The present invention provides a method for screening candidate agents to identify compounds that modulate the hemostatic system. The method of the invention involves a screening medium comprised of stored whole blood, preferably diluted with buffer, to which unrefrigerated platelets have been added. Candidate agents that may inhibit or activate clot formation or clot lysis are added to the screening medium and suitable compounds are identified. The assay provided is physiologically relevant, rapid, inexpensive and allows for large scale screening of candidate agents.