The Biopharmaceutics Classification System (BCS) is the scientific basis for classifying drugs based on their aqueous solubility and intestinal permeability that supports in vivo bioavailability and bioequivalence waivers for immediate-release solid dosage form drugs. One requirement of the BCS is that the permeability method must be validated. In order to accommodate the variety of in vitro/in situ permeability models, the BCS Guidance gives a general framework for the validation requirements, necessitating implemented experimental details to be selected by the applicant laboratory. The objective of this work was to define the parameters for a cell based in vitro permeability method (e.g., cell type, pH, transport direction, time, and concentration) and validate the method to support formal BCS classification of drugs. Twenty reference drugs were selected and permeability values determined using the Madin-Darby canine kidney type II cell line heterologously expressing the human P-glycoprotein transporter (MDCKII-MDR1). A rank order relationship was established between the in vitro permeability value and human intestinal absorption values. This relationship was as predicted and validates the MDCKII-MDR1 permeability method as defined by the BCS Guidance. The final validated in vitro permeability method employs the MDCKII-MDR1 cell line incubated with the Pgp inhibitor GF120918. It is a unidirectional apical-to-basolateral transport assay performed at apical pH values of 5.5 and 7.4 and a basolateral pH of 7.4. Four reference standards (metoprolol, pindolol, labetalol and ranitidine) dosed and analyzed as a single cassette are included in each experiment. A strategy on selection of drug concentrations and on how to deal with problematic compounds (i.e., those suffering from poor mass balance) is discussed.