A cylinder-piston unit, of a needle-free injector, with at least one cylinder accommodating an injection solution, and with at least one piston. The piston is composed of a drive body and of a sealing body. The sealing body is at least one elastic disc, which bears under a radial clamping action against the inner wall of the cylinder. The drive body is a pot-shaped body with a peripheral elastic skirt. At least one area of the skirt bears under a radial clamping action against the inner wall of the cylinder. The cylinder chamber areas located in front of and behind the drive body communicate with each other via at least one recess. The drive body is arranged behind the sealing body in the cylinder, and the skirt faces towards the sealing body. A piston is developed which, at ambient pressure, can be set down on the liquid level of a filled cylinder in a bubble-free, sterile and gas-tight manner and which nevertheless withstands a high compression pressure without leakage during the targeted emptying of the cylinder-piston unit.
