In the writer's previous paper, * he reported his experimental results on the turbidity of pressed-out juices from several plants, for the purpose of diagnosing viroses chemically and physically. The present report deals with the relation of the colloid stability to the turbidity of pressed-out juices from mosaic-diseased roots of radish and from crinkle-diseased potato tubers. The results obtained are summarized as follows. The turbidity of the original pressed-out juices from healthy radish and potato was lower than that from diseased ones. The turbidity of juices kept standing for 8 hours was shown to be lower. In both cases when the radish or the potato was used, the diseased juice was more quickly clarified than the healthy, and the potato juice was found to be lower in its clarifying degree than the radish juice. (Table 1 and 4). The centrifuged juice was found to be lower in its turbidity than the still-held juice, but the tendency toward clarification of the former was similar to that of the latter. (Table 2 and 5). The quantity of precipitates (mainly protein substance) caused by the addition of saturated (NH4)2SO4 solution was proportional to the degree of turbidity demonstrated in the above-mentioned experiments. (Table 3 and 6). From the results obtained, the writer considers that the colloid stability of the pressed-out juice from virus-infected potato or radish is lower than that from healthy ones, and that this is in part due to the lower buffering capacity of the diseased juice.