Abstract: In a time-marking sedimentation process where two or more fractions in a sample containing liquid are to be separated entirely or partially by their specific weights in a tube or corresponding structure, a plug (2) of a hydrophilic plastic substance is brought into contact with the sample. The plug (2) has original dimensions smaller than the inner diameter of the tube (1) and has a specific weight lying between the specific weights of the fractions. The plug is adjusted with respect to its dimensions and/or substance so as to attain a swelling degree sufficient for the plug to jam in the tube (1), to mark the separation line between the fractions (3,4) after a certain predetermined period. Thereafter the position of the plug (2) in the tube is read. The method and device are particularly advantageous in medical connections, especially in the determination of the sedimentation reaction of blood, in which blood corpuscles are separated from plasma.