Abstract: A method and a device for separating the serum included in a certain volume of blood by way of centrifugation. The volume of blood is introduced into a test tube (1) in which a so-called separating body (5; 5') is also present, after which the test tube (1) is centrifugalized. The blood is separated into a heavy phase (11) containing red blood corpuscles and a light phase (12) containing serum or plasma. The separating body (5; 5') is given such a specific gravity that upon centrifugation it enters a position in the boundary layer between the heavy and the light phase (11 and 12, respectively).A (9; 9') stored in the separating body (5; 5') is moved out of this towards the heavy phase (11) and seals between separating body (5; 5') and test tube (1). When the mass (9; 9') is removed from the separating body (5; 5') a negative pressure will arise therein which is utilized to suck-up accumulated leukocytes and trombocytes into the separating body (5; 5').