The argyrophilic proteins of the nucleolar organizer region (Ag-NOR proteins) were specifically localized at the optical level with a modified one-step silver technique performed at 20 degrees C. This method was applied to various materials including cells in smears, chromosomes, semi-thin sections of plastic-embedded cells and sections of paraffin-embedded human pathological tissues. In order to improve the visualization of the silver deposits we tested various modes of imaging, including bright-field, Nomarski contrast, reflected light and combined Nomarski contrast with reflected light. The use of Nomarski contrast is useful to define precisely the phases of mitosis. The use of reflected light, which is based on the ability of silver to reflect incident light specifically, gives images with an improved resolution compared to bright-field.