The anticomplementary activity (ACA) of immune complexes or DNA was measured, then those data were compared with that in SLE sera. 1. High ACA was detected mainly in γ-GI and β-Gl fractions of SLE sera. 2. DNA (especially denatured DNA) showed high ACA, and the treatment with DNase eliminated its ACA. DNA did not bind nonspesifically to normal γ-Gl. 3. ACA in some SLE sera was reduced after DNase treatment. This treatment worked mainly on ACA of γ-Gl fraction. 4. The procedure of complement inactivation which is employed in measurement of ACA, depressed ACA of cryoglobulin, experimental immune complexes and SLE γ-GI fraction. This indicated that heat aggregation of native IgG could not account for high ACA in SLE sera. 5. The addition of enough amount of complement reduced ACA of immune complexes, but a certain level of ACA was still retained. 6. ACA of insoluble immune complex was much higher than that of soluble immune complex. These data may suggest that ACA in SLE sera originates from the circulating immune complexes, especially DNA-antiDNA antibody immune complexes and this ACA detected is the reflection of only a part of ACA caused by immune reaction in vivo.