In this recent study we have investigated the consequences of E. amylovora infection of pear fruits. Inoculations were made with a mixture of pear isolates (Ea 21, 23; at a density of 5 × 108 cells/ml) collected from various growing regions of Hungary. The change of concentration in time of different carbohydrate fractions (mainly glucose, fructose and sucrose) in unripe pear fruits of different cultivars in connection with disease development was measured using the OPLC method. Peroxidase enzyme activity was also measured in the samples during the disease process. Samples (unripe fruit flesh cutouts - 1 cm 0) were taken from the inoculation point and neighboring tissues during seven day intervals. Host responses showed significant differences. We have concluded that among the three well-detectable carbohydrate fractions, glucose and sucrose were the best markers of the disease process. The concentration of glucose was higher in the susceptible uninfected control ('Max Red Bartlett') tissues than in the resistant ones ('Vicar of Winkfield') and it decreased after the infection in a different way. Increase in peroxidase enzyme activity started to occur on the first day after inoculation (DAI) in the susceptible cultivar, while the change was detectable only 2 DAI in resistant fruits. It was a suitable marker of the susceptible and resistant host responses.