Abstract: The paper discribes the labouratory experiment with tree leaves for measuring their disinfecting action on Tubercle bacilli. Tubes containing Tubercle bacilli were kept together with leaves of 20 tree species. The volatile substances secreted by leaves inhibit Tubercle bacilli through the medium of air. In comparison with the control the inhibiting effectiveness of almost all tree species accounts for 100%,that of Lagerstroemia indica L. was calculated to be 86%. The experiment also show that the inhibiting effect increases with the amount of leaves.It can be concluded tentatively that trees are closely related to people's health.