The mode and distribution of hyaluronic acid in Wharton's jelly of the human umbilical cord was studied by the electron microscopic histochemical technique. By means of ruthenium red (RR) staining, RR-positive material was observed as granules approximately 30nm in diameter in the fibroblast of Wharton's jelly. Around the fibroblast, it was observed as fibril structures on the cell surface or as granules approximately 13nm in diameter in the basement membrane-like structure around the cell. In the extracellular matrix, it was observed as granules or amorphous materials on collagen fibrils. Only the 30nm granules in RR positive structures disappeared due to amylase digestion. The other structures were digested by Streptomyces hyaluronidase. On the basis of the above findings, it was concluded that hyaluronic acid is formed as hydrated gel around the fibroblast and collagen fibrils in Wharton's jelly of the human umbilical cord. These results suggest that its structure is one of the factors in maintaining the elastic aptitude of the umbilical cord, to protect from any pressure, such as fetal movement, etc., in the uterine cavity.