Separator float for blood collection tubes

A separator float has a generally cylindrical body and a water swellable band having a selected outer diameter fitted circumferentially around a portion of the generally cylindrical body. The water swellable band is made of a water swellable material so that upon being exposed to water the water swellable band swells to a greater outside diameter. The separator float is preferably used in a blood collection tube and has a specific gravity between specific gravities of a light phase and a heavy phase of the blood. After the blood has been collected and centrifuged the separator float moves to a position between the light phase and the heavy phase and the water swellable band is exposed to water in the blood causing the band to swell and create a seal. Thereafter, the serum or plasma as well as white blood cells, lymphocytes, monocytes and platelets can be removed from the blood collection tube.