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SUMMARY The paper constitutes a general introduction to the biology and application of bacterial plasmids. The current definition of a plasmid is given as well as historical background of the discovery of plasmids and early plasmid research. Plasmid structure and size range are presented. The rules recommended for proper plasmid naming and classification are given. The methods of plasmid identification in bacterial cells and its isolation in the form of purified DNA preparations are summarized, followed by the scheme of a typical procedure for plasmid characterization.