1. Resistance to Apple Mildew (Podosphaera leucotricha) is shown to be inherited quantitatively and apple varieties vary in the resistance contribution which they pass on to their progeny. 2. The mean relative resistance values of progenies of given varieties correspond closely irrespective of whether they are the result of selfing or outcrossing. 3. It is shown that resistance in progenies is correlated with the resistance reactions of the parents. 4. A statistical analysis of the data, using methods applicable to an incomplete diallel cross, has enabled the contribution of each parent to resistance to mildew of its progenies to be calculated. Of the cultivated varieties, Beauty of Bath and Lord Lambourne gave the highest contributions and Merton Russet and Late Orange the lowest. This method of analysis allows the estimation of the mean resistance of the progenies of varietal crosses in the absence of complete observational records.