Method and apparatus for mixing a specimen (e.g. a biological fluid specimen) in an upstanding container having an internal mixer with a cavity defined by an upstanding annular wall. The annular wall of the mixer is engaged by a movable holder, which is elevated along with the engaged mixer sufficiently to prevent contact of the mixer with the container during mixing. The mixer is then moved to mix the specimen. The holder, which may include an expanding collet that fits within the cavity of the mixer, preferably rotates the mixer to stir the specimen. When mixing is complete, the holder is disengaged from the mixer. A controller coordinates all actions of the holder.