Eight flax cultivars were evaluated for powdery mildew 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 flax cultivars was studied by two methods. In the first method, the antiserum of the infected whole plants of cultivar Giza 8 interacted against antigens of seeds from the healthy flax cultivars. In the second method, antiserum of O. lini alone interacted against the same antigens after the elimination of Giza 8 antibodies from the antiserum by the reciprocal preceptitation. In the first method, regression analysis indicated that the common antigens between the infected Giza 8 and the healthy cultivars accounted for 86% of the explained (model) variation in severity of powdery mildew on these cultivars. In the second method, regression analysis indicated that the common antigens between O. lini alone and the healthy cultivars accounted for 76% of the explained (model) variation. The first method was superior to the second method for two reasons. First, it did not require the application of the reciprocal precipitation, which greatly simplified the test. Second, this method gave higer coefficient of determination (R²) value, which indicated that it was more reliable for predicting resistance to powdery mildew.