A simple and efficient procedure for the rapid isolation of plasmid DNA free of chromosomal DNA and with only minor contamination with RNA is described. The protocol is a modification of the boiling method described by Holmes and Quigley [(1981) Anal. Biochem. 114, 193-197.] and utilizes C18 reverse-phase silica beads for final concentration and purification of plasmid DNA. The entire procedure can be carried out in 1 day and does not require the use of phenol or cesium chloride gradients, which require considerable labor and may sometimes cause nicking and lower recoveries of supercoiled DNA. The plasmid DNA obtained by this method retains biological activity, is supercoiled, and is suitable for restriction and DNA sequence analysis.