Complement activation occurs in many pathological conditions. Assays to evaluate the presence and extent of this activation may be limited by being qualitative, time-consuming, or radioactive. We have recently devised an enzyme-linked immunoassay that quantitatively measures the presence of the complement activation product C3d in plasma. The assay is rapid: Results can be available within 8 hours of submission. Intra-assay variation was low (4.9%) as was interassay variability (8.7%). This assay was then used to demonstrate that patients with systemic lupus erythematosus (SLE) have increased levels of circulating C3d as compared to those of normals (p less than 0.001). This assay may be useful to demonstrate continuing complement activation and inflammation in patients, even those without clinical symptoms.