1. Platelet survival and an index of the localization of platelets in the kidney were studied in patients with the proliferative nephritis of systemic lupus erythematosus, either focal or diffuse, and in control subjects. Platelet survival was reduced in patients with proliferative lupus nephritis, more in those with diffuse rather than focal renal involvement. 2. The index of renal platelet localization in patients with diffuse proliferative nephritis suggested an intrarenal platelet consumption not found in other groups. 3. A patient with the classical platelet autoantibody disease, idiopathic thrombocytopenic purpura, also showed reduced platelet survival but localization of platelets was in the spleen rather than the kidney. 4. Intrarenal platelet consumption in diffuse proliferative lupus nephritis may be an epiphenomenon of pre-existing scarring or platelet aggregation secondary to immune complex-formation, which contributes to the progressive sclerosing lesions of this form of nephritis.