The spectrum of cellular targets in the autoimmune diseases is both large and varied and includes among the nuclear components the so-called Sm and RNP antigens associated with systemic lupus erythematosus. The use of immunoaffinity chromatography with dual specificity for the Sm and RNP antigens has allowed for their substantial purification from rabbit thymus in parallel and in quantity. In lieu of a functional assay, the use of a counterimmunoelectrophoresis assay provided a sensitive and rapid means of monitoring the distribution of the two antigens during purification and ensured the isolation of complexes containing the components required for antigenicity. The resulting purified complex consisted of nine polypeptides having molecular weights of approximately 9000 to 44,000 and two small RNAs of similar size. However, limited proteolysis of the isolated complex suggested that most of these polypeptides were not actually required for antigenic activity. Unlike Sm in crude thymus extracts, purified Sm was RNase sensitive. Thus, one of the major diagnostic criteria used to distinguish Sm and RNP antigens in crude extracts was shown to be invalid for purified material, suggesting that both antigens from rabbit thymus are actually ribonucleoprotein complexes.