A method for preserving viral particles comprises: (i) providing an aqueous solution of one or more sugars, a polyethyleneimine and said viral particles wherein the concentration of polyethyleneimine is 15 &mu；M or less based on the number-average molar mass (Mn) of the polyethyleneimine and the sugar concentration or, if more than one sugar is present, total sugar concentration is greater than 0.1 M； and (ii) drying the solution to form an amorphous solid matrix comprising said viral particles.
