An apparatus for separating e.g., a fibrin monomer from blood includes a container (110) with a reaction chamber defined by an outer wall for receiving plasma and a means for supplying the chamber with an agent for converting the fibrinogen content of the plasma into a non-cross-linked fibrin polymer. The apparatus includes a device for centrifuging the plasma and the agent within the chamber to a degree sufficient for separating the fibrin polymer from the plasma, depositing the polymer on the outer wall of the chamber and expelling the remaining plasma from the chamber. The container (110) includes means for supplying the chamber with a solvent for dissolving the fibrin polymer. The apparatus includes a measuring device (130) for measuring the amount of the deposit of the fibrin polymer on the outer wall of the container (110) and a control unit (131) for controlling the addition of solvent in response to the amount of polymer.