A novel method for large-scale plasmid preparation is described. Crude extracts are subjected to acidic phenol extraction to remove any contaminants present in the aqueous phase. The supercoiled plasmid DNA, which preferentially remains in the organic phase and inter-phase, is extracted back into the aqueous phase with 1.5 M TRIZMA base, from which it is precipitated. The resultant plasmid DNA is highly pure and satisfactory for any subsequent procedures. The method is extremely economical and takes only 3-4 h.