The anticomplementary activity (ACA) of the sera from systemic lupus erythematosus (SLE) patients was measured by the modified Wasserman-Levine's method, and the following results were obtained. 1. The sera from SLE, liver cirrhosis, Hashimoto disease and rheumatoid arthritis patients revealed high ACA. 2. In SLE, ACA of the sera with high concentration of γ-Gl or IgG was significantly higher than those of the sera with low concentration of it. 3. There was no correlation between ACA and serum complement level in SLE, but ACA was rather high in hypocomplementemic sera. 4. ACA of the sera that showed the shaggy pattern was especially high among the sera showing various ANF patterns, and ACA correlated with the level of anti-DNA antibody in SLE. 5. ACA was significantly high in sera with high level of rheumatoid factor in SLE. 6. ACA in SLE varied with activities of lupus nephritis. These data may suggest that positive ACA shows the exsistence of the circulating immune complexes in SLE sera, and the measurement of ACA is important to know the prognosis of SLE.