An improved method for depositing metal particles on a marker which catalyzes the reduction of metal ions from a physical developer comprising a solution of metal ions, a molar excess of complexant in respect to the metal ions and a reducing agent. Said method being preferably employed for the detection of one or more components of an aggregate formed between a least one specific binding agent and its corresponding bindable substance by labelling at least one component of said aggregate with a marker and contacting said aggregate with a physical developer, whereby under influence of the marker a metal particle is formed which can be detected. Further the invention also relates to solutions and test-kits adapted for carrying out the above mentioned method.