An immunoassay method comprising applying an aqueous solution containing theanalyte antigen to one end of a multi-zoned test strip device (10) such that the solution moves along the strip (10)by capillary action. The zones are arranged so that thesolution (a) first contacts and reconstitutes dry, diffusible labelledcomponent comprising colloidal gold conjugated to an antib-ody specific for said analyte antigen (20) and then (b) contacts andreconstitutes dry, diffusible biotinylated second antibody spe-cific for said analyte antigen (18) such that a diffusible, dispersed sandwichreaction product forms. The reaction product diffusesalong the strip (10) with the solution and into a zone containing capturecomponent consisting of a latex and avidin complex (22)which avidin collects the reaction product by means of reaction with itsbiotin moiety. Thus, gold particles are collected and con-centrated in the detection zone (22) for visual determination.
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