The invention relates to a closure cap (1) for dialyzers which is suitable for a so-called in-line sterilization. For this purpose the closure cap (1) comprises two cap parts (2, 3) of which the first cap part (2) is adapted to be mounted on a dialyzer flange (4) while the second cap part (3) is arranged movably in a receiving compartment (5) of the first cap part (2) between an open position and a closure position. In an open position passage of a sterilization medium is possible so that all the parts, sealing and guide surfaces can be reliably sterilized. In the closure position the second cap part (3) is pushed into a sealing position within the first cap part (2) so that contamination of the dialyzer after removal from the sterilization apparatus can be prevented with certainty.