Abstract: An adapter 4 fitted in a tube holder 1 is composed of four flexible extensions 23 integral with the forward end of the adapter, and each extension includes a slanted portion 27 and a gripping portion 28 which may have a stopping lug 30 formed on the latter portion. An evacuated tube 3 is closed with a plug 15, which compresses an elastic sheath 12 covering the needle and is pierced by it together with the sheath, when inserted into the adapter 4 for taking a blood sample. The slanted portions 27 are elastically swingable outwardly to render the plug 15 coaxial with the holder 1. The gripping portions 28 are also capable of flexing following the swung slanted portions so that their forward ends are transitionally displaced centrifugally and then restored to their straight position to hold the plug 15 such that even if its outer diameter is considerably smaller than the inner diameter of the holder 1, the sampling needle 2 can penetrate the plug through its central zone and the plug is not "kicked back".