In sera from patients with systemic lupus erythematosus and Sjögren's syndrome (SS), antibodies are present in high titres to small cellular proteins which may be involved in RNA processing. In this study we used affinity chromatography with antibodies isolated from patients sera to purify two cellular antigens; SS-A (Ro) from human spleen and SS-B (La) from rabbit thymus. Both antigens co-purified as a molecular complex from human thymus. The protein components were acidic though SS-A was resistant, and SS-B sensitive to trypsin. On polyacrylamide gel electrophoresis SS-A migrated as a single 55K polypeptide and SS-B as 40-45K polypeptides with a 29K degradation product. All of these polypeptides were reactive in the western blot with their respective antibodies from characterized sera and similar components in SS-B were demonstrated in tissue extracts from a range of mammalian species. These purified antigens represent important reagents for the investigation of the aetiology of some types of autoimmunity.