Potatoes to be used for seed purposes must reach a certain standard of freedom from various diseases, notably the virus diseases, severe mosaic and leaf-roll. Normally, stocks to be used for seed purposes are inspected during the growing season, with a view to grading them into stock seed categories or otherwise. This is usually a fairly laborious process and, at times, may not be really accurate. Apart from this, other stocks not previously graded as stock seed may be required for seed purposes and, once the tops of the plants have died down, there is no method whereby the presence of virus infection can be proved or disproved, except by grafting, a process which takes a few months to complete. The object of the present investigation was to explore the connection between the biochemical activity of tubers and the incidence of virus infection. Since a principal line of attack had been suggested by earlier studies on the reducing value of the ascorbic acid in tubers, attention was directed to a more complete study of the concentration and distribution of ascorbic acid occurring under different conditions. The correlation of biochemical activity with the incidence of virus infection would be an important step in the control of disease. The importance of such a relationship is at once obvious from a consideration of the following points. Crops could be graded scientifically and the personal error of the inspector could be eliminated. The stocks could be graded at any period of the nine months when the tubers are out of the ground instead of the very short interval of time when the plants are actively growing in the field. The method could also be used, in addition to the method used at present, if crops, which had not been inspected, were required under exceptional circumstances for seed purposes.