SUMMARY Properly purified preparations of poliovirus particles are practically free of o-galactose and N-acetyl-D-glucosamine. These sugars, however, are found in crude virus preparations, obtained by high and low speed sedimentation and by CsCl-gradient fractionation from infected HeLa cells. They are separated from the virus particle by extraction with chloroform and subsequent isopycnic sedi- mentation of the virus preparation. Since o-galactose and N-acetyl-o-glucosamine are regular constituents of glycoproteins, the absence of significant amounts of these sugars demonstrates the lack of glycoproteins in poliovirus particles. Radioactive monosaccharides are recommended for the detection of cellular impurities in carbohydrate-free viruses. A simple and rapid method for the purification of large quantities of poliovirus by 'precipitation' with polyethylene glycol, resuspension in CsC1 solutions, extraction with chloroform and isopycnic sedimentation is described.