The dimethylmethylene blue assay for sulphated glycosaminoglycans has found wide acceptance as a quick and simple method of measuring the sulphated glycosaminoglycan content of tissues and fluids. The available assay methods have lacked specificity for sulphated glycosaminoglycans in the presence of other polyanions, however, and have not discriminated between the different sulphated glycosaminoglycans. We now describe a modified form of the dimethylmethylene blue assay that has improved specificity for sulphated glycosaminoglycans, and we show that in conjunction with specific polysaccharidases, the dimethylmethylene blue assay can be used to quantitate individual sulphated glycosaminoglycans.