In experiments on the rate of translocation of the potato viruses YN and YO in the varieties Bintje, Eigenheimer, Dore and Sirtema it was found that both viruses infect tubers of Bintje much more readily than those of Eigenheimer. The tubers of the four varieties proved to be infected by PVYN to a higher degree than by PVYO. Tubers of the variety Dore became infected by PVYN to a very high degree, whereas the PVYN infection of those of the variety Sirtema reached the same level as Bintje tubers. Mature plant resistance could be demonstrated with respect to both viruses, but it was much more pronounced in the experiments with PVYO than in those with PVYN.