In the development of an embryonic chick limb, temporal and spatial transitions have been observed in the type of collagens. Undifferentiated mesodermal cells have been reported to synthesize Type I collagen, and Type II collagen appears in accordance with the onset of chondrogenesis in limb development [ 1 ] . With collagen, proteochondroitin sulfate is one of the major components of the extracellular matrix. Two types of proteochondroitin sulfate have been found in the cartilage of chick embryo, based on sucrose gradient separation [2]. The occurrence of proteochondroitin sulfates has not been reported, prior to chondrogenesis, only that of chondroitin sulfates have been observed [3-61. We have been interested in identifying the type of proteochondroitin sulfate synthesized by the chick embryo prior to chondrogenesis. We report here that the chick embryo does synthesize a new type of proteochondroitin sulfate, before chondrogenesis takes place. It is distinctly different from two proteochondroitin sulfates, hitherto found in the embryonic chick cartilage, with respect to its sedimentation profile on sucrose gradient, amino acid composition and the length of mucopolysaccharide chain.