Aerosil A-300 and GF/F (GF/C) filters were used for purifying plasmid DNA and intact RNA as well as for the recovery of DNA fractionated on agarose gels. In the presence of guanidinium thiocyanate Aerosil A-300 and GF/F (GF/C) filters selectively bind nucleic acids, while proteins, agarose polysaccharides and salts are washed off. Nucleic acids desorbed from the Aerosil and filters are available immediately as substrates: DNA for restriction analysis, ligation and sequencing; RNA--for RT-PCR.