Passive hemagglutination (PHA) and hemolysis (PHL) tests using chromium chloride-treated sheep red blood cells were developed to detect and measure the anti-DNA antibodies. Sonication of native DNA was found to prevent the incidence of non-specific agglutination. Sheep red cells were coated with double-stranded DNA (dsDNA) which had been sonicated and treated with nuclease S1 to digest the single-stranded regions in the DNA. The specificities for dsDNA-coated cells were checked by inhibition studies in PHA test and plaque assay. In clinical studies fairly close correlations were found between the antibodies to DNA and the activity of the disease in patients with systemic lupus erythematosus (SLE). Complement-fixing antibodies were detected in most of SLE patients with active lupus nephritis, but rarely in those in remission. Anticomplementary activity seemed to be negligible in PHL test. These tests are simple and may be useful to the diagnosis and the management of SLE.