A tested person blows into an alcohol-breath measuring device through the mouthpiece. The mouthpiece must be kept under hygienic conditions and must be disposable, thus designed for single use by a tested person. The mouthpiece is an integral part injection molded of polypropylene, having a mouth portion, and a portion connecting to the measuring device and extending through the bottom of the mouth portion into the interior thereof. The tested person takes the mouthpiece between his lips and exhales through the connecting portion into the device. A limiting flange on the outside of the mouth portion limits the penetration thereof in the mouth and keeps saliva from the surface intended for holding. The mouthpiece may accommodate a check valve. A valve insert received in the mouth portion holds a valve plate which under the blow, elastically disengages from apertures in the bottom of the insert and applies against these apertures again as soon as the blowing ceases or the tested person inhales.