Purpose. In healthy corneal epitheliur i HGF is detectable only at the epithelial celi surface. After corneal epithelial woinding HGF expression in keratocytes is upregulated. HGF is thought to modulate epithelial cell proliferation, motility and differentiation; it can also alter polarity (Balkovetz et al, in press). We reported that polarity affects the susceptibility of epithelial cells to P. aeruginosa virulence mechanisms. This study examined the effect of basolateral cell surface exposure to HGF on susceptibility to P. aeruginosa. Methods. Epithelial cells were grown on Transwell filters. In some experiments the basolateral cell surface was treated with 0.1 u,g/ml HGF for 72 h prior to bacte 'ial interaction assays. In other experiments, two clones of MDCK. cells with different selective defects in sorting of proteins and lipids to the correct cell membrane wei e used. Susceptibility of cells to invasion by P. aeruginosa strain 6294 (gentamicin survival assays) and cytotoxicity by strain 6206 (trypan blue exclusion) was compared to untreated parent MDCK cells. Results. HGF caused large increases n the susceptibility of cells to both bacterial invasion and cytotoxicity by P. aeruginosa strains (>10-fold). Clone C13, was equally susceptible to bacterial invasion as the control Clone C15, but was more susceptible to cytotoxicity. Clone n/J, .vas also more susceptible to cytotoxicity, but was less susceptible to bacterial invasion than the control Clone II/G. Conclusions. Factors that upregulite HGF in the cornea could increase the susceptibility to infection via a direct effect of HGF on the epithelial cells. This also shows -without chelating calcium to disrupt tight junctions -that epithelial cells are polarized in their susceptibility to P. aeruginosa. Results obtained with cell clones that missort apical and basolatenil membrane components suggest that cellular susceptibility factors for invasion and cytotoxicity are not the same, even though both appear to be associated with the be solateral cell surface.