An immunofluorescence test for antibodies to native DNA, using the kinetoplast of Crithidia luciliae as substrate, has been assessed in comparison with the Farr precipitation technique, on a total of 395 sera from sixty-three patients with systemic lupus erythematosus, 185 other hospital patients and sixty healthy controls. The immunofluorescence test appears to have great specificity as a diagnostic test for SLE, though lacking the sensitivity of the Farr technique. Like the latter, it is altered by immunosuppressive treatment, and in patients with SLE nephritis on immunosuppression it does not show good correlation with activity of renal disease. Its specificity and simplicity nevertheless make it a valuable clinical test.