The relationship between mannitol crystallization during freeze-drying and its effects on stabilizing protein structures was studied using lysozyme, bovine serum albumin, ovalbumin, beta-lactoglobulin and lactate dehydrogenase as model proteins. FT-IR analysis of the protein secondary structure indicated perturbation of both alpha-helix and beta-sheet regions in freeze-drying without cosolutes, whereas the proteins retained most of their native structure in co-lyophilization with sucrose. Mannitol protected the protein structure to different degrees depending on the crystallinity. The combination of mannitol with potassium phosphate buffer reduced the mannitol crystallinity and the structural changes occurring during freeze-drying, whereas mannitol by itself showed little stabilizing effect. Heat-treatment of the frozen solutions at -10 degrees C resulted in a higher mannitol crystallinity and a smaller stabilizing effect in freeze-drying. The secondary structure perturbation was mostly reversible in rehydrated solutions. The varied structure-stabilizing effects of mannitol paralleled its effects on maintaining lower concentrations of enzyme activity during freeze-drying. These results confirm the contribution of molecular interactions between amorphous excipients and proteins (e.g. hydrogen bonding) to structure stabilization during freeze-drying.