Glyceollins are phytoalexins produced in soybeans from their isoflavone precursor daidzein. Their impressive anticancer and glucose normalization effects in rodents have generated interest in their therapeutic potential. The aim of the present studies was to begin to understand glyceollin intestinal transport and metabolism, and their potential effects on P-glycoprotein (Pgp) in Caco-2 cells. At 10 and 25 μM, glyceollin permeability was 2.4±0.16×10(-4) cm/sec and 2.1±0.15×10(-4) cm/sec, respectively, in the absorptive direction. Basolateral to apical permeability at 25 μM was 1.6±0.10×10(-4) cm/sec. Results suggest high absorption potential of glyceollin by a passive-diffusion-dominated mechanism. A sulfate conjugate at the phenolic hydroxyl position was observed following exposure to Caco-2 cells. In contrast to verapamil inhibition of the net secretory permeability of rhodamine 123 (R123) and its enhancement of calcein AM uptake into Caco-2 cells, neither glyceollin nor genistein inhibited Pgp (MDR1; ABCB1) up to 300 μM. There was no significant change in MDR1 mRNA expression, Pgp protein expression, or R123 transport in cells exposed to glyceollin or genistein for 24 h up to 100 μM. Collectively, these results suggest that glyceollin has the potential to be well absorbed, but that, similar to the isoflavone genistein, its absorption may be reduced substantially by intestinal metabolism; further, they indicate that glyceollin does not appear to alter Pgp function in Caco-2 cells.