The permeability of human bronchial epithelial cells (16HBE14o -) to radiolabelled insulin ([125I]insulin) formulated in the absence or presence of two different saccharide-containing permeability enhancers was investigated. In the absence of either enhancer, mannitol permeability and transepithelial electrical resistance (RTE) remained essentially unaffected for the duration of a 2-h experiment. Addition of either 0.125% tetradecylmaltoside (TDM) or 1% dimethyl-β-cyclodextrin (DMBCD) to the apical surface of cells resulted in increased mannitol permeability and decreased RTE, suggesting a loosening of cellular tight junctions and a concomitant increase in paracellular movement. Addition of [125I]insulin to the apical side of 16HBE14o- cells in the absence or presence of 1% DMBCD resulted in little or no [ 125I]insulin movement to the basolateral chamber or degradation in the apical chamber. However, in the presence of 0.125% TDM, the amount of intact [125I]insulin remaining in the apical chamber was substantially decreased, while [125I]insulin and 125I-labeled fragments were recovered on the basolateral side of the cells after 2 h. These findings provide evidence that the loosening of the tight junctions between cells achieved with DMBCD is not sufficient to stimulate transepithelial insulin movement, whereas exposure to 0.125% TDM causes an increase in [125I]insulin permeation and degradation. © 2003 Elsevier B.V. All rights reserved.