Sample tube racks having retention bars to retain sample tubes in the racks during processing of the contents of the sample tubes are described. An example rack for holding sample tubes includes a sample tube carrier having an elongated body and walls defining apertures. Each of the apertures is configured to receive a respective one of the sample tubes. The walls define elongated openings, each of which corresponds to a respective one of the sample tubes and extends along at least a portion of a length of the respective sample tube, and the elongated openings enable viewing of information on the outer surfaces of the sample tubes. The example rack also includes an elongated retention bar to be pivotally coupled to one end of the sample tube carrier. The retention bar has openings, each of which is positioned over a respective one of the apertures, and the openings are dimensioned to prevent removal of the sample tubes from the sample tube carrier through the retention bar.