The objective of the invention is a crystallizing method for macromolecules, especially proteins and polypeptides, in which selected polysaccharides of biological origin, such as alginate, pectin, dextrin or chitosan and hydrolysates thereof, are used as reagents. Sedimentation of the crystals can be prevented and thus the uniformity of the product contributed with the method. The method can be used to prepare new crystal forms of the polypeptide and to improve the stability of crystals.