Delta G (DG) is the biologically active fragment of Zonula Occludens Toxin (Zot), an absorption enhancer, that reversibly opens the tight junctions of epithelial and endothelial cells in the small intestine and brain. This study evaluates the possible use of DG in enhancing the oral bioavailability of macromolecules using large paracellular markers as model agents. The transport of [ 14 C]Inulin and [ 14 C]PEG4000 was evaluated across Caco-2 cells with DG (0, 100, 180 mg/ml). The apparent permeability coefficients (P app ) were calculated. The in vitro toxicity of DG (180 mg/ml) was assessed. Sprague Dawley rats were dosed intraduodenally (ID) with the following treatments: [ 14 C]Inulin or [ 14 C]PEG4000 (30 mci/kg) w/o DG (720 mg/kg)/protease inhibitors (PI). Blood was collected and plasma was analyzed for radioactivity. DG (180 mg/ml) increased [ 14 C]Inulin and [ 14 C]PEG4000 P app by 82.6 and 24.4%, respectively, without any toxicity. After ID administration with DG/PI, C max and AUC were significantly (p < 0.05) increased for both Inulin and PEG4000. However, Inulin displayed greater enhancement ratios in vitro and in vivo. This study suggests that DG may be used to enhance the oral bioavailability of macromolecules (e.g., proteins) after coadministration through modulation of paracellular transport.