Experimental cucumber and tomato plants were cultivated under glasshouse conditions with or without PGPR. Young plants were infested with spider mites (T urticae - cucumber and T cinnabarinus - tomato). Leaves were analysed for protein and amino acid concentration. In infested leaves of the cucumber cultivar, susceptible to spider mites (Corona), an important decrease in protein content occurred in both bacterized and nonbacterized plants. Amino acid content was higher in mite infested leaves and the presence of PGPR did not influence this phenomenon. The content of proteins was also studied in the leaves of the less susceptible cucumber cultivar (Aramis). Spider mite feeding caused a small decrease in their concentration. The presence of bacteria in the root system caused an increase of soluble proteins in the leaves. In infested leaves of the highly susceptible tomato cultivar (Rornatos), cultivated without bacteria, an evident increase of amino acid content was found opposite to plants with PGPR. The protein concentration was also increased in injured leaves of this cultivar but the presence of bacteria in the root system of mite infested plants caused a decrease of these compounds in their leaves. Injured foliage of the less susceptible tomato cultivar (Sionka) had a lower protein concentration in bacterised plants as compared to those nonbacterised and both controls.