An association between the complement system and immune complex glomerular disease in humans has long been recognized. In fact, much of our early understanding of the immunochemistry of complement activation developed with the study of acute and chronic glomerulonephritis (1). This manuscript will examine associations between glomerulonephritis and the three complement components encoded within the major histocompatibility complex: C4, C2, and factor B (B). The mechanisms by which deficiencies or polymorphisms in these components can mediate disease will be examined.