Resistance of six modern clones of the Aigeiros populars to Marssonina populi has been investigated and their biochemical changes in the course of disease, including the activities of polyphenasc (PPO) and peroxidase (PO), and the variations of PO isozymes have been compared. Test results revealed that I-63, I-69 and I-72 were resistant, "blome de garonne" was susceptible, I-214 and P15A were mast susceptible. In the course of disease the activities of PPO in resistant colnes were higher than in susceptible ones. During the initial stage of infection, the activities of PPO in all clones increased. The activity of PO in the infected susceptible clones were higher than in the resistant ones. There were more PO isozyme bands in the suseeptible clones than in the resistantones.