Corroding medium and its influence on chemical composition of the upper layer of tool steels. Tests included influence of the corroding media of pine, beech and oak wood extracts on chemical composition of the upper layer of cutters made of highly alloyed and low alloyed steel. It was demonstrated that only low alloyed steel undergoes corrosion and that the extract from pine wood has got the highest corrosive invasiveness, while the one from oak wood has got the lowest.