Studies on purification and properties of potato virus M (PVM) were carried out using an isolate found in British Columbia. The narrow host range of the virus was confirmed, and no new susceptible species was discovered. Potato cultivars failed to develop symptomps even in plants produced by tubers of inoculated plants, but none was immune. An attempt to demonstrate transmissibility of the virus by plant contact was unsuccessful. In undiluted potato sap the virus had a thermal inactivation point (TIP) of 65 to 70°C, and a longevity in vitro (LIV) of 2 to 4 days. The dilution end point (DEP) was 10⁻⁴. The LIV and DEP of the virus in tomato sap were similar to those in potato sap. Crude sap diluted to 10⁻¹ induced more lesions on Red Kidney bean than undiluted sap. An efficient purification procedure for PVM was developed. The virus was purified from leaves of potato (Solanum tuberosum L.), by extraction with 0.5 M borate buffer, pH 7.8, clarification with ammonium sulfate (20%), and concentration with ammonium sulfate (30%). Further concentration was carried out by high speed centrifugation followed by polyethylene glycol (PEG 6000) precipitation and high speed centrifugation. Final purification was by sucrose density gradient centrifugation. The yield obtained from this procedure was 3.7 to 4.1 mg per Kg of infected leaves. The purified preparations contained rod-shaped particles 651 nm normal length and 13.4 nm average width. The particles had an A₂₆₀/A₂₈₀ratio of ]^3' an A max/A min ratio of 1.24, a maximum ultraviolet light absorption at 260 nm, a minimum absorption at 245 nm, and a buoyant density in CsCl of 1.304 (suggesting an RNA content of 6.2%). The molecular weight of the protein subunit was about 39,300 daltons. / Land and Food Systems, Faculty of / Graduate