An apparatus for transporting racks of sealed sample tubes, mixing a sample material contained within the tubes, and aspirating samples from the tubes. The apparatus includes a car which runs along a rail. The car drives racks containing the sample tubes into a mixer channel disposed above and running parallel to the rail. The mixer channel is mounted to rotate about an axis parallel to the rail to mix samples and position the tubes to be aspirated. A push pin mounted adjacent to the mixer channel extends into the mixer channel and urges individual tubes against an aspirator assembly mounted on the opposite side of the mixer channel. An input queue is disposed proximate the input side of the rail and an output queue disposed proximate the output side of rail. The input queue supports the racks and feeds them into position to be driven by the car mechanism into the mixer channel. The output queue receives racks after being aspirated and feeds them aft to make room for subsequent racks driven by the car from the mixer channel. The racks include a lateral front wall, a lateral rear wall and a horizontal bottom wall connecting the front wall to the back wall. The racks also include a plurality of vertical side walls connected to the front wall and the back wall to define a plurality of tube receptacles. The bottom wall includes a plurality of equidistantly spaced apertures, each of which is centrally located within a tube receptacle to provide the push pin access to the tubes. The bottom wall also includes a plurality of ribs extending from the front wall to the back wall to be engaged by the car. Biasing means are disposed within each of the tube receptacles to support sample tubes therein.