Summary.Experiments on the purification of the virus of yellow mosaic of tomato are described and discussed. Vinson and Petre's methods of purification of the virus from infectious juice and subsequent elution with phosphate solution were slightly modified in these experiments. It was found that there was no evidence that the virus could be recovered in a crystalline form and that viruliferous material always contained traces of organic nitrogen. This virus was found to be active over a wide range ofpH, viz. from 2.0 to 10.5. At the extremes of the scale, the excessive acidity or alkalinity was toxic to the inoculated leaves and adjustments had to be made before inoculation. Different protein precipitants were used in an attempt to free the virus from the proteins, and electrolytic methods were also tried. Proteolytic enzymes were employed on the purified virus juice but the results were rather unsatisfactory. Difficulty was experienced in ensuring that the effect of some reagents was on the virus and not on the tissues of the test plants.