1. Experimental infections with virus S on a number of varieties showed that serological tests performed on the plant failed to prove a new infection with certainty. Such new infection remains symptomless, while invasion of the tuber proceeds relatively rapidly. 2. During the last three years 67 varieties were tested : 22.4 percent were found to be of low, 47.8 percent of medium, and 29.8 percent of high resistance. 3. The rate of tuber contamination of a primary infected plant increased from resistance stage 1 with 37.9 percent virus infection to 83.0 percent in resistance stage 9. With the degree of resistance the portion of noninfected plants decreased, that of 100 percent infected plants increased. Symptomatologically, there was almost total latency after secondary infections; only in a few cases were non-uniform and varying symptoms observed. 4. The potato plant has a well defined mature plant resistance to virus S. There are practically no differences in the degree of infection conferred to older, unequally resistant varieties, so that inoculation must be done at an early age when resistance differences are to be recorded. 5. The resistance tests of 1964-1967 used different methods of inoculation. Rubbing the virus-containing sap on one leaf of each sprout resulted in a low degree of infection, inoculation by spray gun in a high one. From a comparison of the different years and methods of infection one may conclude that the resistance of a variety can be reliably reproduced if one half of the plants are inoculated by rubbing, the other half by spray gun. A medium degree of infection and good differentiation of the varieties can thus be obtained. 6. Only with late manual inoculation of a highly susceptible variety did early lifting result in a remarkable decrease of infection. As the result of mature plant resistance such an effect was prevented in potatoes of medium susceptibility.
