Antinuclear antibody (ANA) titers in complement immunofluorescence (CIF) and conventional indirect immunofluorescence (IIF) staining were established and CIF/IIF ratios of ANA titers calculated. The following properties of conjugates seem to be indicated for determinations of CIF/IIF ratios: (1) anti-C4 conjugate dilutions used for CIF staining of ANA should be adjusted to contain ½ unit of antibody/ml; (2) anti-IgG conjugates used for this purpose should be adjusted to ¼ unit/ml; (3) for determinations of CIF/IIF ratios of ANA titers, the two conjugates used should have comparable F/P ratios or optical sensitivity ratios. Using such a system a preliminary double-blind study of 54 selected sera from patients with various collagen diseases was undertaken. After completion of titrations and decoding, it was found that 11 sera of patients with a diagnosis of systemic lupus erythematosus (SLE) all had ANA titers of 200 or more and all fixed the C4 component of complement. The CIF/IIF ratio of these 11 sera fell in the range of 1:6 to 1:1. Of the remaining 43 sera 4 fixed complement. Of these, 2 had antinucleolar antibodies rather than true ANA. Of the other 2 sera which fixed C4, one was from a case of discoid lupus erythematosus and another from a patient with rheumatoid arthritis. Their CIF/IIF ratios were 1:8 and 1:21, respectively. Tests for ANA patterns of 25 coded sera in another laboratory yielded peripheral staining with 3 of 4 SLE sera and with none of the other sera included.