The invention relates to a device for separating a bodily fluid, in particular blood platelet concentrate, into fractions under centrifugal force. The device comprises a flexible container system which comprises at least a first container for the fluid to be separated and a second container for a separated fraction, wherein the containers are mutually connected using a connecting conduit for a fluid, wherein a widened portion is arranged in the connecting conduit between the first and second container, wherein during use a detectable separating layer is created in the widened portion. The invention also relates to a flexible container system intended for the device, a method for separating blood platelet concentrate into fractions under centrifugal force, and a blood platelet concentrate with low leukocyte content.
