Incorporation of [³⁵S]sulphate by cultures of matrix-free cells from chick embryo sterna in the presence of the glutamine analogue 6-diazo-5-oxo-L-nor-leucine (0.58 mm) was inhibited in a time-dependent manner to <15% of that in control cultures after 2 h.  Characterization of the major cartilage proteoglycan synthesized under these conditions showed that it contained few, if any, normal-sized chondroitin sulphate chains, and only about half of the normal complement of substituted serine residues.  Subsequent addition of D-glucosamine hydrochloride resulted in a time-dependent recovery of [³⁵S]sulphate incorporation to 90% of control cultures after 2 h. but restored the chondroitin sulphate chains to normal size within 15 min.