Immunopathological evidence suggests that activation of the alternative pathway of complement (AP) is involved in membranoproliferative glomerulonephritis (MPGN) and in immunoglobulin A nephropathy. In this report we describe an AP dysfunction-associated factor that was isolated from the serum and urine of a patient with hypocomplementemic MPGN. Extensive glomerular deposits of C3, properdin, and of the terminal complement components were observed in the kidney of the patient. In her serum the AP hemolytic activity was virtually absent. When mixed with fresh normal serum, the patient's serum induced a 96% C3 conversion during a 30-min incubation at +37 degrees C. This activity was found to be due to a circulating factor that by immunochemical characterization proved to be a 46-kD monoclonal immunoglobulin lambda light (L) chain dimer (lambda L). Purified lambda L, but not control lambda or kappa L chains from patients with L chain disease, activated the AP in a dose- and ionic strength-dependent manner. Functionally, lambda L was differentiated from C3 nephritic factor (an autoantibody against the AP C3 convertase, C3bBb) by its inability to bind to and stabilize the C3bBb enzyme. Instead, lambda L was observed to interact directly with the AP control factor H. Thus, lambda L represents a novel type of immunoglobulin-related AP-activating factor with the capacity to initiate alternative complement pathway activation in the fluid phase.