1. The herbal products Natto K2, Agaricus, mistletoe, noni juice, green tea and garlic were investigated for in vitro inhibitory potential on P-glycoprotein (P-gp)-mediated transport of digoxin (30 nM) in differentiated and polarized Caco-2 cells. 2. Satisfactory cell functionality was demonstrated through measurements of assay linearity, transepithelial electric resistance (TEER), cytotoxicity, mannitol permeability, and inclusion of the positive inhibition control verapamil. 3. The most potent inhibitors of the net digoxin flux (IC(50)) were mistletoe > Natto K2 > Agaricus > green tea (0.022, 0.62, 3.81, >4.5 mg ml(-1), respectively). Mistletoe also showed the lowest IC(25) value, close to that obtained by verapamil (1.0 and 0.5 microg ml(-1), respectively). The IC(50)/IC(25) ratio was found to be a good parameter for the determination of inhibition profiles. Garlic and noni juice were classified as non-inhibitors. 4. This study shows that mistletoe, Natto K2, Agaricus and green tea inhibit P-gp in vitro. Special attention should be paid to mistletoe due to very low IC(50) and IC(25) values and to Natto K2 due to a low IC(50) value and a low IC(50)/IC(25) ratio.