Previous work (Hashimoto et al., (1994) Biosci. Biotech. Biochem. 58, 1345) revealed that a sweet pepper extract enhanced the tight-junctional (TJ) permeability of a human intestinal Caco-2 cell monolayer. In the present study, the substance which modulated the TJ permeability was chromatographically purified from the extract. The active substances were identified as capsianosides A-F, diterpene glycosides. Treatment of the cells with capsianoside F, the most active compound, decreased the cellular G-actin content by 40% and increased the F-actin content by 16%. The effect of capsianoside F was significantly suppressed by disturbing the cytoskeletal structure with cytochalasin D at a low dose (50 ng/ml). These results suggest that capsianosides affected the cytoskeletal function by modulating the reorganization of actin filaments, by which the TJ structure and permeability were changed. The possible involvement of a PKC inhibition in the mechanism of an increase in TJ permeability is also suggested.