The membrane potential of black lipid membranes (BLM), formed from 20 ppm didodecyl phosphite in (3 + 2) octanedodecane containing 1% cholesterol, has been measured across a BLM separating solutions of 1:1 salts (NaCl and KCl) at 25 degrees C in the absence and presence of a 1.5-kG static magnetic field, The membrane potential decreased when the magnetic field was applied perpendicular to the BLM, These data have been used to evaluate the effective fixed charge density and the permselectivity for ions of the BLM. The magnetic effects on the membrane potential of the BLM are quantitatively interpreted by the estimated parameters from the preliminary results measured under the magnetic field : the 20-30% increase in the effective charge density of the BLM and the associated promotion of the permselectivity of the BLM.