Abstract: The invention relates to a method for fluid separation of whole blood as a mixture of liquids into individual, differently-colored blood constituents, which blood is packed in flexible containers, in particular bags, wherein the bags are connected to one another with an at least partially light-transparent connection, in particular a flexible tube, and wherein the blood constituents are forced to flow from one container through the light-transparent connection into another container, in particular for the separation of concentrated thrombocytes from buffy coat, as well as to a device for performing the method.