Studies using an adapted immunofluorescence technique (IFT) on Crithidia luciliae to determine the complement fixing ability of antibodies to dsDNA in relation to disease manifestations, i.e., nephritis, have yielded conflicting results. To establish the relevance of these determinations, we studied sera containing antibodies to dsDNA from 64 patients with systemic lupus erythematosus (SLE), and found that anti-dsDNA of 52% of these sera had the ability to fix complement. SLE patients with nephritis demonstrated a much higher incidence of complement fixing anti-dsDNA (83%) than patients without nephritis (17%, p less than 0.01). On the other hand, patients with nephritis also had higher titers of anti-dsDNA (mean 1:400) than patients without nephritis (mean titer 1:75; p less than 0.01). A clearcut correlation between anti-dsDNA titer and complement fixing anti-dsDNA titer (p less than 0.01) was observed which obviously disturbs the correlation between nephritis and complement fixing anti-dsDNA. Comparing matched sera from patients with nephritis and patients without nephritis with the same antidsDNA titer, we found no difference in complement fixing anti-dsDNA. In the IFT used to measure complement fixing anti-dsDNA, incubation of the Crithidia slides with patients' serum was followed by an incubation with fresh normal serum which served as a source of complement. We observed that this incubation with fresh normal serum resulted in elution of anti-dsDNA antibodies from kinetoplast DNA. This elution was caused by IgG present in normal serum.