Disclosed is hylan, a chemically modified hyaluronan preparation characterized by the presence of small amounts (0.0002-0.05% by weight) of aldehyde cross-linking groups covalently bonded to the hyaluronan molecular chains. Also disclosed is an improved method of obtaining hylan comprising treating hyaluronan in situ in animal tissues containing same with a treatment mixture including an aldehyde which is reactive towards hyaluronic acid and the proteins contained in the animal tissue. Treatment is effected at temperatures below about 16.degree. C. at very high pH, e.g., 8-14 whereby the overall yield is increased while maintaining the high molecular weight of the hylan.