Solutol HS-15 is a permeability enhancer. A general decline in cell metabolic activity is seen for all three tested cell lines as the concentration of Solutol HS-15 solution is increased. Results demonstrate that Solutol HS-15 promotes the apical-to-basolateral translocation of a model protein (insulin) across Calu-3 monolayers when applied at 37°C. Furthermore there is no statistically significant increase in insulin permeability in the presence of Solutol HS-15, relative to the respective control, at 4°C. Solutol HS-15 treated cells show a statistically significant increase in permeability towards lower molecular weight permeants (5 to 22 kDa) and an absence of a statistically significant increase in permeability of proteins with larger molecular weight, namely albumin and IgG (P>0.05 relative to control).