Abstract: A microtiter stick for use in performing immunochemical tests has a solid plastic body and a row of shallow wells (26). Each well is in the form of a truncated hemisphere, the bottom of which is an optically clear, circular, transparent, concave lens surface (26'). The remainder of the truncated hemisphere is stippled to render it translucent thereby to diffuse light and to increase the effective contact surface area of the well. A wall (31) surrounds the upper surface to create a reservoir so that all the wells (26) may be filled at one time. Lens surfaces (36) are formed in the bottom of the stick in optical alignment with the lens surface (26') in each well to create a planar-concave, clear, transparent lens at the bottom of each well.