We describe the methodology used for quantifying and characterizing porphyrins in various tissues and in excreta, in the diagnosis and monitoring of the therapeutic modulation of biochemical disease activity in a 53-year-old white man who has a rare form of familial porphyria cutanea tarda with bone marrow rather than hepatic expression of the disease. Liquid-chromatographic and thin-layer chromatographic analyses of the patients's urine and skin showed predominantly heptacarboxylic porphyrin and uroporphyrin, whereas his stool and bile contained isocoproporphyrin and coproporphyrin as the major products. The data reflect defective uroporphyrinogen decarboxylation. Both analytical methods gave quantitatively similar results for urinary and fecal porphyrins. A triple-lumen perfusion study of samples procured both at the ampulla of Vater and 15 cm downstream provided data for porphyrins excreted in the bile and their reabsorption in the small intestine. We evaluated: suppression by hypertransfusion of bone marrow overproduction of porphyrins and reduction of enteral absorption of porphyrins by orally administered charcoal (Acta Char) and cholestyramine.