Proteoglycan heterogeneity was studied during the in vivo differentiation of embryonic chick limb cartilage. Recently, it has been shown that during the differentation of limb cartilage the proportion of the aggregated form of proteoglycans increases whereas the unassociated monomeric forms decrease, and this has been related to the synthesis of two link proteins at a specific stage of differentation. In this study it is suggested that the appearance of the aggregate formation is also due to synthesis of a stable hyaluronic acid binding region of the core protein. Thus, it can be concluded that differential gene expression for these proteins takes place as a differentiation phenomenon.