The technique of silver staining of the active nucleolar organizer regions is discussed. The specificity of the technique, the mechanism of the reaction, the nature and possible functions of argyrophilic protein(s) are considered. Special attention is paid to the fact, ascertained elsewhere, that the silver deposits in the nucleolus are related to the structure of "activated" (potentially active) ribosomal chromatin, rather than to the process of transcription itself. In this concern the supposition that the argyrophilic protein could interact with rDNA maintaining its non-nucleosomal conformation is under discussion. The possibility that the argyrophilic protein could be a component of the residual nucleolus is also taken into account. Whatever the function of the argyrophilic protein may be, this protein can be considered as a reliable marker for the active and "activated" ribosomal genes.