When sap from tobacco, infected with potato X virus, was dialyzed against 0.05 Mand then against 0.005 Msodium citrate, no aggregation of the virus was detected after ultracentrifugation. Flow birefringence measurements were made to compare virus preparations obtained by ultracentrifugation of untreated sap and of sap that had been treated with various buffers. Only versene and citrate were found to prevent aggregation. The influence of pH in the range 5.7–7.3 was very slight. Possible explanations of this effect are discussed.Nitrogen and phosphorus content and the sedimentation constant of the purified, non-aggregated virus are reported.