Abstract: The present invention is directed to a new assay procedure and improved solid phase assay devices for use with a so-called physical developer. The invention provides a new assay procedure for detecting the presence of an analyte in a fluid sample using a tracer having a metal label which signal is enhanced by applying a physical developer. The assay is performed with a device having a support for a test area and having attached thereto a binder specific for the analyte to be detected. The assay comprises the steps of directing the fluid sample and the tracer through a porous medium into contact with the test area at a controlled flow rate to allow binding of the analyte with the binder and the tracer to the analyte, directing a physical developer through said porous medium to amplify the signal generated by the tracer, and separating the device to allow the test area to be read. The invention further relates to devices and immunoassay test kits for carrying out the improved immunoassay procedure.