This invention relates to the detection of biological substances and is particularly though not exclusively applicable to labelling techniques employing antibodies or other specific binding agents. Thus the invention consists in a method for the detection of a biological substance of interest (optionally in an environment of biological cells) in which a marker substance is linked specifically, directly or indirectly, to the substance of interest, the marker substance being selected to act as a catalyst in a subsequent process of reduction of silver using a developing solution. |