The degradation of hyaluronic acid induced by ascorbic acid and the effect of synovial fluid proteins, such as ceruloplasmin, transferrin, and albumin, were investigated on the basis of the elution volume and the molecular weight of hyaluronic acid using high-performance gel permeation chromatography. Hyaluronic acid was degraded to less than one-third of the original molecular weight in the range of the physiological concentrations of ascorbic acid. Synovial fluid proteins protected against the ascorbate-dependent degradation of hyaluronic acid at their physiological concentrations. It is suggested that the inhibitory activity of ceruloplasmin mainly depends on the ferroxidase activity and that of transferrin is probably due to iron binding property.