Abstract: An apparatus (10) and method for use in a vertical flow-through assay process is characterised by applying pressure to a sample to force the sample through a reaction/capture membrane (12), to which one or more ligands are bound, at a controlled rate. Typically, the method includes a pre-incubation step in which the sample and a detection analyte typically an antibody bound to colloidal gold or a fluorescent tag bind together The pre-incubation step typically takes place in a chamber (26) spaced above the capture membrane (12). The base of the chamber is defined by a porous hydrophobic frit (34) typically formed from polyethylene. It is preferred that the chamber (26) is defined by the upper part of a cylinder (24) extending from a seal (30) compressing the reaction membrane (12) against an absorbent pad (28). The seal (30) has the effect of compressing tie reaction membrane and preventing wicking of the sample in the lateral direction outside of the circular seal.