The development of aqueous two-phase systems for plasmid purification from Escherichia coli cell lysates requires a reliable DNA quantitation method. Plasmid DNA was quantified by fluorescence using PicoGreen nucleic acid stain. Linearity was obtained up to 40 ng plasmid ml(-1). Two polyethyleneglycol (PEG)/salt systems were studied, PEG 600/K2HPO4 and PEG 300/K2HPO4. The average plasmid recovery was 41% in the bottom phase of the first system and 35% in the top phase of the second system. This method has proved to be simple and reproducible.