Abstract: To allow different gel bed heights in a chromatographic separation column (1), a plunger (6) is displaceable within the column (1) into contact with the gel bed (5) by means of a shaft (7) extending through an end piece (3) at the end of the column (1), said end piece (3) having a locking means for locking the shaft (7). According to the invention said locking means comprises a resilently biassed locking piece (8) having grooves extending essentially perpendicular to the longitudinal direction of the shaft (7) which grooves engage corresponding grooves on the shaft (7) in the normal position of the locking piece (8). In its normal positon, the locking piece (8) is adapted to allow displacement of the shaft (7) only towards the gel bed (5). Moreover, the locking piece (8) is manually operable from its normal position to allow free displacement of the shaft (7).