A collaborative series of experiments has been conducted under the auspices of EPPO to investigate methods of assessing resistance to potato cyst nematodes. Two of these experiments, designed to study centre × clone interactions, are reported. Six partially resistant clones derived from S. vernei were assessed in pot tests at centres in Germany and the United Kingdom using common sources of tubers and two populations of G. pallida. Analyses of the data showed that for each population the main source of variation was between clones. However there were significant centre × clone interactions, which accounted fore. 5% of the total variation. Despite this and the differences in absolute multiplication rates observed at the different centres the clones were ranked similarly by each centre and population. These results support the proposal that quantitative resistance be assessed in relation to standard resistant clones.