Antibodies in the sera of patients with systemic lupus erythematosus reacted with a nuclear acidic protein called Sm antigen, and these antibodies were used as reagents to identify Sm antigen in preparative fractionation procedures. DNA affinity chromatography showed that Sm antigen was associated with nuclear protein fractions which had DNA-binding capacity. Evidence was also presented that Sm antigen showed preferential binding for single-strand DNA over double-strand DNA. These studies demonstrate that spontaneously occurring anti-nuclear antibodies in disease states may be used to study the properties of cellular proteins which are present in trace amounts.