A field trial was conducted in 2002/2003 and 2003/2004 growing seasons at Giza Agricultural Research Station to evaluate the reactions of ten flax cultivars to powdery mildew (PM) disease, In general, the tested cultivars could be divided into four distinct groups, i.e., highly resistant (Ottawa 770 El. Dakota, and Bombay), resistant (Cass, Wilden, and Clay), susceptible (Koto and Marshall), and highly susceptible (Cortland and C1. 2008). The cultivars showed consrderable variatlon in disease severity (DS) ranged from 3.69 on Bombay to 100% on CL 2008. lsozymes of malate dehydrogenase (MDH). peroxidase (PRX) and esterase (EST) of cultivar seed were separated by polyacrylamide gel electrophoresis (PAGE), and the obtained banding patterns were visualized by using three staining systems. Data for PM severity (dependant vanable). and amounts of isozymes (independent variables or predictors) were entered Into a computerized stepWise multiple regression analysis, Using the predictors supplied by stepwise regression three models were constructed to quantify PM severity. Coefficient of determination (R ) values of the models were 98.67, 83.46, and 83 46% for MDH, PRX and EST, respectively. These results indicate that PAGE of isozymes may provtde a supplementary assay to greenhouse and field tests to distinguish quantitatively between PM resistant or susceptible cultivars.