Abstract: Methods and apparatus for the extraction of DNA from a suspension of cells are described. The methods utilise a hollow membrane filter to separate DNA from cellular debri after lysis of cells. The suspension of cells can be a suspension of cultured cells or cells contained in a body fluid such as blood. An ion-exchange step can be included in methods so that purified DNA is provided. The apparatus (1) comprises a vessl (2) in which cells can be cultured. The vessel has a hollow membrane filter (5) associated with an end thereof. The described methods can be used for extracting genomic DNA from cells but are particularly suitable for extracting plasmid DNA from microorganisms.