Abstract: The invention relates to a closing cap for infusion and transfusion bottles. The inventive cap is provided with a flanged cap which encompasses a flanged neck of the bottle, secures a closing plug in the bottle and is provided with a middle recess that is closed by the middle region of a plastic lid in such a way that a ring zone (21) of the plastic lid (2) supports the edge of the middle recess from behind. At least one radially extending desired folding line (I) of the plastic lid (2) is embodied. Said line is formed by thinning the material. The aim of the invention is to indicate when said cap has been used. In an advantageous embodiment, the desired folding line (I) opens into a circumferential weakened area (S) that extends at a radial distance (y) from the center (z) of the plastic lid (2).