Abstract: A blow mandrel for the production of hollow bodies in a hollow blow mold, particular for the production of filled bottles sealed ready for shipment, from a length of tubing of a thermoplastic resin, having an axially displaceable blow pipe adapted to be surrounded by an edge of the mold opening. The blow mandrel comprises the aforementioned blow pipe and, at its mouth section, a sealing ring protruding outwardly beyond the circumferential surface of the blow pipe and extending in closed form in the circumferential direction of the pipe upon blowing in the region of the edge of the opening of the blow mold. The sealing ring has, on its side turned toward the mold cavity, an annular groove open in the direction of the blow-mold cavity.