Eight flax cultivars were evaluated for powdery mildew (PM) resistance under field conditions in 2001/2002 and 2002/2003 growing seasons. The tested cultivars could be divided into two distinct groups. The first group included the resistant cultivars Dakota, Wilden, and Williston Brown, while the second one included the susceptible cultivars Cortland, Linore, C.I.2008, Giza 7, and Giza 8. The difference in disease severity was highly significant (p < 0.01) between any cultivar belonged to the first group and any cultivar belonged to the second group. The antigenic relationship between Oidium lini and the fax cultivars was studied by double diffusion (DD) and immunoelectrophoresis (IE). In each method, the antiserum of infected whole plants of cultivar Giza 8 interacted against the antigens of seeds from the healthy flax cultivars. In DD test, simple linear regression analysis indicated that the common antigens between the infected Giza 8 and the healthy cultivars accounted for 86% of the total variation in severity of PM on the tested cultivars. On the other hand, R² value of IE regression model was very low (15%) and its F. value was nonsignificant. Therefore, DD may provide a supplementary assay to field trials to distinguish between PM resistant or susceptible genotypes quantitatively.