Abstract: An apparatus is provided for use in assays for the detection of a bindable target substance in a liquid sample. The apparatus comprises an elongate reaction membrane strip which is porous and liquid-permeable, the upper surface of the membrane strip having receptors thereon capable of binding to the target substance; an absorbent material located adjacent to the lower surface of the reaction membrane strip; and a container means for the reaction membrane strip and absorbent material, including a top wall defining a fluid port in the form of an elongate slot adjacent the membrane strip and including a fluid seal means disposed at the periphery of the fluid port defining a seal between the container top wall and reaction membrane strip. In one embodiment, the receptors on the membrane strip correspond to Western blot proteins. An assay for the detection of bindable target antibody in a liquid sample employing the apparatus is also described.