Abstract
A method of analysing the reactions of a range of cultivars of a host to a range of isolates of a pathogen, where the reactions are recorded as compatible or incompatible, is presented. The method allows the minimum number of genes required to explain the reactions under a gene-for-gene model to be calculated, and gives an answer which does not depend on the order of presentation of the data, or on any rearrangement which may take place during analysis. The method is illustrated using data from the reactions of cultivars of Lactuca sativa (lettuce) to isolates of Bremia lactucae (downy mildew).