A study was performed to determine some properties of the potato virus M (PVM) in crude sap and in pure preparations. In crude sap the virus was shown to have a dilution end point value of 10-4 longevity in vitro particles sedimented as one peak with a sedimentation coefficient value of 162s. The buoyant density in cesium chloride was 1.304 gm/cm3. Electrophoresis in polyacrylamide gels showed that the virus possessed only one protein subunit with a molecular weight of 39,300 daltons.