A disposable cuvette is molded of rigid transparent plastic resin that is impervious to liquids to be handled within it. The cuvette has a rectangular cross section. Identical side walls extend between upper and lower ends of each cuvette, the upper end being open for reception of liquids and the lower end being closed. Recessed optical surface areas at the lower end are perpendicular to one another for optical testing of the cuvette contents. An elongated cartridge holds a stack of abutting cuvettes to facilitate their handling.