THE use of paper electrophoresis has enabled several investigators to establish the presence of acid mucopolysaccharide components in various tissue extracts1,2 and urine3. Two such substances, staining with toluidine blue, were found in human aortic tissue4; concentrated human urine contained three components which stained with alcian blue5. The latter all appeared to migrate more rapidly than the albumin fraction, with the fastest component having the same mobility as chondroitin sulphate A. In the present work, which is primarily concerned with the electrophoretic characteristics of various body fluids in agar, a component was noted in the perchloric–phosphotungstic acid precipitate of both serum and pleural fluid which possessed staining and electrophoretic properties similar to those of acid mucopolysaccharide in urine.