A simple, scalable method for purification of plasmid DNA is described. Plasmid DNA was released from Escherichia coli JM109 by lysis (1% SDS, 0.2 M NaOH). Then a neutralization solution (3 M sodium acetate buffer, pH 4.8) was added to precipitate genomic DNA and protein. After the clarification of the lysate, the supernatant was placed in a multicompartment electrolyser separated by ultrafilter membranes to remove the remaining contamination (RNA, genomic DNA and protein). A recovery of 75%+/-2% of total plasmid DNA was obtained after 60 min electrophoresis with a field strength of 8 V cm(-1) using cells at 30 g l(-1) (quantified by dry cell weight). Genomic DNA, RNA and protein were undetectable in the purified plasmid DNA solution.