High molecular weight hyaluronan (M(r) 400,000) obtained from human umbilical cord was depolymerized by sonication for 10 h into small molecules and finally into molecules of constant size (M(r) 11,000). The molecular size of the depolymerized hyaluronan was unaltered even under different conditions of sonication. After sonication, the main sugar residues at the reducing and non-reducing termini of depolymerized hyaluronan were N-acetylglucosamine (86%) and glucuronic acid (98%), respectively. Hyaluronans derived from rooster comb (M(r) 1 x 10(6)) and Streptococcus zooepidemicus (M(r) 1.2 x 10(6)) were depolymerized into molecules of different but characteristic sizes by sonication. On the other hand, neither chondroitin sulfate nor glycogen was depolymerized by sonication. These results suggest that high molecular weight hyaluronan may have some weak linkages related to N-acetylglucosamine in the chain, which are extremely sensitive to sonication. At present, however, the nature of these linkages is still unclear.