Abstract: A specimen tray assembly for use in an automatic analyzing system for analyzing a plurality of specimen within the tray. The assembly includes a container tray having a plurality of microcuvettes arranged in a spaced apart grid-like pattern. A cover member is adapted to seat over the top surface of the container tray, the cover member including a system for automatically centering the container tray relative to the cover member to provide proper seating of the cover member on the container tray.