Abstract: The invention herein is directed to a method for the visualization of the product of the reaction of a specifically binding substance and a corresponding bindable substance. The reaction product is formed during an assay procedure for the detection and/or determination of one of the components of the reaction. The assay involves the use of a component labeled by a sol of a non-metallic element, such as selenium and tellurium, and the use of a physical developer comprising a metal compound, such as silver lactate and silver nitrate, mercuric chloride and a copper sulfate, to strengthen or reinforce a specific signal, thereby improving detectability.