A potentially fruitful but hitherto relatively unexplored application of the analytical ultracentrifuge is the investigation of interactions in pharmaceutical polymer drug carrier systems. In this study we investigate the interactions of the cationic material chitosan with gastric mucin: 1) The physicochemical properties of gastric mucin and chitosan (highly deacetylated chitosan “Sea Cure + 210”) in solution are summarised; 2) We described how the gastric mucin (from pig) can be purified by a combination of preparative isopycnic density gradient ultracentrifugation and gel chromatography, and how its purity and structural integrity can be checked by analytical isopycnic density gradient ultracentrifugation (on a MSE Centriscan 75 Analytical Ultracentrifuge) and on line GPC/MALLS (gel permeation chromatography coupled on-line to a multi-angle laser light scattering photometer); 3) Using co-sedimentation experiments with the appropriate controls in an XL-A Ultracentrifuge (absorption optics) and an MSE Mk II Analytical Ultracentrifuge (Schlieren optics) a definite interaction between chitosan and pig gastric mucin was demonstrated, with complexes sedimenting faster than 1000 S, depending on the amount of the mucin used.