A partially quaternized chitosan derivative, N-trimethyl chitosan chloride (TMC) (degree of quaternization 12.28%), was synthesized and the effects of this novel polymer on the permeability of intestinal epithelial cells, using Caco-2 cell monolayers, were investigated and compared with those of chitosan hydrochloride and chitosan glutamate. Transepithelial electrical resistance (TEER) measurements at pH 6.20 revealed that all these polymers (0.25-1.5% w/v) caused an immediate and pronounced lowering in TEER values in the order chitosan hydrochloride (84% reduction after 2 h incubation) > chitosan glutamate (60% reduction) > TMC (24% reduction) at 0.25% w/v concentrations. At higher concentrations (up to 2.5% w/v), TMC was able to decrease the TEER further. Similar results were obtained in transport studies, using the hydrophilic radioactive markers, [14C]-mannitol (MW 182.2) and [14C]-polyethylene glycol 4000 (PEG-4000, MW 4000). Large increases in the permeation of these markers were found. The transport of [14C]-mannitol was increased 34-fold (chitosan hydrochloride), 25-fold (chitosan glutamate) and 11-fold (TMC) at 0.25% w/v concentrations. Further increases in the permeation of both markers were found at higher concentrations of TMC. Due to its quaternary structure, TMC is better soluble than the other chitosan salts, and its higher solubility may compensate for its lesser effectivity at similar concentrations. It is also soluble at pH 7.40, where the chitosan salts are insoluble and therefore ineffective. No deleterious effects to the cells could be demonstrated with trypan blue exclusion studies and confocal laser scanning microscopy (CLSM). CLSM confirmed that these polymers increase the transport of large hydrophilic compounds (using the fluorescent markers FD-4, MW 4400 and FD-20, MW 19,600) through opening of tight junctions to allow for paracellular transport. It is concluded from this study that the charge, charge density and the structural features of chitosans and chitosan derivatives are important factors determining their potential use as absorption enhancers.