To preserve the size and transfection potential of polymer-plasmid complexes. Freeze-drying and freeze-thawing were used for stabilization of these complexes. The concentration of the sugars is an important factor affecting both the size and transfection capability of the complexes after freeze-drying and freeze-thawing. However, the type of lyoprotectant (sugar) used is of minor importance. It is also shown that when damage to polymer-plasmid complexes occurs, it results from the drying process but is not due to the freezing step.