Ninety-seven sera, 58 from patients with SLE and 39 from patients with other autoimmune rheumatic diseases were tested for anti-ds-DNA antibody activity by ELISA, Farr, and Crithidia Lucilliae assays. Fifty-six per cent of the sera were positive by at least one method. Eighty per cent of the SLE population was positive by ELISA, 39% by Farr Assay and 33% by the Crithidia assay. Crithidia assay exhibited the greater specificity (100%) followed by the Farr Assay (97%) and the ELISA (80%). Sera positive by all methods showed a significantly higher mean value of the ELISA rates, than sera positive only by ELISA (p less than 0.001). When the SLE sera were analyzed according to disease activity, it was shown that ELISA and Farr Assay correlated well with the lupus activity index (LAI) (r less than 0.001). The SLE sera with the higher anti-ds-DNA concentration (sera positive by all 3 methods) did not correlate with LAI when tested by the Farr Assay (0.05 less than p less than 01) in contrast to the ELISA values, which correlated very well (p less than 0.01). Our results indicated that the ELISA is the most sensitive method with reasonable specificity as well. In addition, the ELISA anti-DNA values correlate with the clinical activity of lupus, independently of the anti-ds-DNA levels in the sera. The latter should be attributed to the fact that this method detects all the heterogenous population of anti-ds-DNA.