Peroxidases and their isoenzyme pattern were studied in two groups of young plants : one resistant to P. tabacina and the other susceptible to P. tabacina during the stages of installation and development of the parasite. When plants were cultivated on Knop's medium (resistant genes masked) blue mold brought about quite slight variations in the peroxidase activity and the increase was evident only at the time of fungus fructification. On the other hand, when they were fed mainly magnesium (resistance genes expressed) a strong increase in peroxidase activity was evidenced in the two varieties as well as a localization of the enzyme around the infected sites. Rather more important differences were noted in plants resistant to the disease than in those susceptible to it. Cytochemical tests showed that these were due to the host and not to the fungus. Peroxidases seemed to be related to the defence mechanism of the plants against different types of aggression of various origins, whether the plants were sensitive or resistant to P. tabacina. On the other hand, the rate, intensity and localisation of their response, implies that they contribute to the expression of resistance to P. tabacina.