Direct immunofluorescence of tissues derived from patients affected with SLE demonstrates antibodies bound to the extracellular matrix (ECM). In the present work we have tested whether such antibodies are found in the serum and urine of lupus patients and mice. We found that the urine of patients with active SLE and of MRL/lpr/lprmice contains antibodies that bind ECM and that a major target for these antibodies is the 200kDa light chain of laminin which is one of the matrix components. The level of the anti-ECM, anti-laminin antibodies correlates with disease activity.