MDCK (a canine kidney cell line) has been transfected with human genes that encode for efflux transporters, producing modified cell lines that overexpress human transporters. Two such modified cell lines are very useful in assessing test drug efflux potential: 1) MDCK-MDR1 overexpressing human P-gp, and 2) MDCK-BCRP overexpressing human BCRP. The cell lines are grown to form a monolayer that provides a barrier to test compounds between donor and receiver wells, such that most compounds may only traverse by permeability or active transport. This models the barrier between the intestinal lumen and blood supply from the hepatic portal vein. Papp is determined, and compounds that are Pgp substrates are identified.