Abstract: An immunoassay method comprising applying an aqueous solution containing the analyte antigen to one end of a multi-zoned test strip device such that the solution moves along the strip by capillary action. The zones are arranged so that the solution (a) first contacts and reconstitutes dry, diffusible labelled component comprising colloidal gold conjugated to an antibody specific for said analyte antigen and then (b) contacts and reconstitutes dry, diffusible biotinylated second antibody specific for said analyte antigen such that a diffusible, dispersed sandwich reaction product forms. The reaction product diffuses along the strip with the solution and into a zone containing capture component consisting of a latex and avidin complex which avidin collects the reaction product by means of reaction with its biotin moiety. Thus, gold particles are collected and concentrated in the detection zone for visual determination.