Abstract: An improved method for isolating a liquid sample from a specimen container employing an elongated cylindrical tube, sized for insertion into the specimen container, and having a hollow neck formed at one end of the tube. A thin, flexible circular disc projects from the neck for slidable sealing engagement with the internal cylindrical wall of the specimen container. A shoulder is formed on the neck adjacent to the circular disc for engaging and supporting at least a portion of the disc to maintain the seal during insertion and release it during withdrawal of the tube. A removable sample measuring cup on the other end of the tube provides control of an air vent in the tube and instantaneous and easy transfer of the collected sample. The container also desirably includes a filter disposed in said neck aperture to limit the fluid flow from the tube.