Sera from 173 patients with systemic lupus erythematosus (SLE) were examined for antibodies to extractable nuclear antigens (ENA) by counterimmunoelectrophoresis (CIE). Antibodies to four major constituents were identified and their frequencies noted as follows: anti-RNP (28%), anti-Sm (10%), anti-Ro (24%) and anti-La (9%). In addition to these a number of uncharacterised antigens were also noted. Using the CIE method we found that in cases where anti-Sm was present it was always accompanied by anti-RNP. Associations between these two antibodies have been noted previously, and many theories have been put forward in explanation. In this study a simple interpretation of the results is offered based on current knowledge of the molecular structure of Sm and RNP antigens.