We have prepared a new type of anion exchanger, which effectively discriminates between RNA and plasmid DNA. The material is based on a Sephacryl S-500 HR matrix provided with quartenary amine anion-exchange groups. A distinguishing feature of the beads is that a thin (2-3 microm) outer layer of the beads lacks ion-exchange groups. In the synthesis of these beads the vinyl groups in the outer layer of vinylalkyl substituted Sephacryl S-500 HR beads are reacted with bromine. The resulting layer of bromoalkyl groups are hydrolysed, creating an inert outer layer of hydroxyalkyl groups. Finally, bromination and trimethylamine reactions of the inner vinyl groups provide the beads with a core of cationic groups. Large plasmid molecules will not bind to such beads since they are too large to enter the pores and therefore cannot come into contact with the charged matrix in the inner parts of the beads. RNA and protein molecules present in a cleared lysate, on the other hand, readily enter the pores and become adsorbed. A two-column strategy was developed for plasmid purification (recombinant pBluescript, 5.9 kilo base pairs, kbp). The first column was packed with the restricted access anion-exchanger beads (lid beads) and the second column with normal ion-exchange material (same ligand density as the lid beads). Diluted (3x), cleared lysate was pumped through the tandem columns. The first column was subsequently disconnected from the system and the purified plasmid adsorbed on the second column was eluted in a concentrated form (6x) and with 89% recovery. The two-column procedure removed 99.5% of the RNA and 96% of the proteins.