Abstract: A separating and filtering device for effecting sealed separation of blood into a light phase and a heavy phase within a tubular fluid sample container comprising a cup-shaped piston having a closed bottom and a permeable side wall. The piston has axially spaced first and second sealing lips adapted to sealingly engage with the inner surface of the tubular container, and the first sealing lip is deflectable to permit passage of the lighter phase past this sealing lip on movement of the cup-shaped piston downwardly, through the lighter phase, while preventing the heavier phase from such passage.