The transport of pharmaceuticals through silk fibroin membranes has been investigated. The membrane was prepared from Chinese cocoon. The permeability coefficients of five kinds of pharmaceutical, i.e. 5-fluorouracil (5FU), L-(+)-ascorbic acid (Vc), resorcinol (Res), sodium phenolsulfonate (SPS) and benzyltrimethylammonium chloride (BTAC), were measured in the pH range from 3.0 to 9.0. The silk fibroin membrane was an amphoteric ion-exchange membrane composed of both weak acidic and weak basic groups. Membrane-potential measurements revealed that the isoelectric point of the membrane was about pH 4.5. The positively charged pharmaceutical, BTAC, was excluded from the membrane below the isoelectric point. On the other hand, the negatively charged one, SPS, had a permeability that decreased as the pH increased. The neutral molecule, Res, however, penetrated through the membrane independently of pH value over the measured pH range. In the case of 5FU and Vc, the permeability coefficients decreased steeply above pH 7 and pH 4, respectively, because 5FU and Vc were negatively charged above pH 7 (pK(a) = 8.0) and pH 4 (pK(a) = 4.25), respectively.