System, method and bag for centrifugal separation of two or more fractions from a composite liquid, for example blood, comprising a centrifuge with one or more cassettes (16) with an internal chamber adapted for receiving an elongated bag (1) containing the liquid which is to be separated, whereby at least one cassette (16) comprises one or more long, narrow constrictions of the internal chamber which are arranged transverse to the separation direction, and means (8, 12) arranged at these constrictions for clamping of the bag (1) after separation. The system according to the invention is characterized by means (4, 4') for stretching the bag (1) laterally immediately adjacent said constrictions. The bag (1) can be arranged to be sealed with help of one or more pairs of bars (8, 12; 15; 24, 25) at the boundary layers for the various fractions. These pairs of bars can be arranged within and/or outside said cassette.