Abstract: A process is provided for isolating and purifying biologically active RNA from a biological sources containing RNA, DNA and other cellular materials. The process involves contacting the RNA-containing source with particles comprising siliceous material, such as finely-divided glass, in the presence of a binding solution comprising concentrated, acidified chaotropic salt. Under these conditions, RNA, but not DNA, binds selectively to the siliceous material, and can be separated easily from the other components of the sample. Preferably, the selective binding process is applied to biological cells containing RNA of interest. Intact cells are disrupted by exposing them to a lysing solution comprising, as its main component, concentrated, acidified chaotropic salt. The RNA is then isolated and purified from the lysate using the selective binding process of the invention.