A procedure for the purification of viroid RNA from tomato plants is described which yields up to a milligram of viroid RNA of gel electrophoretic homogeneity within 2 days. This technique is at least three times as fast as previous methods and is generally applicable to other RNA species. Plant material was homogenized and phenol extracted. In a Cs2SO4 density gradient, viroid RNA together with low-molecular-weight RNA, was separated from large single-stranded RNA, DNA, polysaccharides, polyphenols, and other compounds. The separation is based on the differences in the buoyant density and on the selective precipitation of large single-stranded RNA in Cs2SO4. Further purification of viroid RNA was achieved by HPLC over a weak anion exchanger linked to silica gel of optimized pore size. The elution was carried out by a salt gradient with complete exclusion of divalent metal ions. The procedures were applied to whole plants, leaves, stems, roots, cells, and protoplasts. The yields of nucleic acids at the different steps of purification are given for leaves, stems, and roots.