Renal biopsies were performed on 11 patients considered clinically and histologically to have pre-eclampsia. Immunofluorescent studies with fluorescein-labelled anti-IgG, IgA, IgM, and IgE complement, albumin, fibrin, and fibrinogen were carried out on the tissue obtained. Significant correlation was obtained between the clinical severity of the disease and the density and pattern of IgM and IgG deposition. Complement was found in glomeruli in severe cases, while complement deposition in the walls of afferent and efferent arterioles was a constant finding. These findings support the concept that an immunological mechanism may be responsible for the renal lesions in pre-eclampsia. If immunity does play a part in the pathogenesis of pre-eclampsia, possible mechanisms include the involvement of histocompatibility antigens and cross-reactivity of fetal and maternal tissues. Renal fibrin deposition in pre-eclampsia may be secondary to an immune process, an occurrence well-described in other forms of glomerulonephritis in man and other species.