Since sulfur dioxide is the most serious air-pollutant in Japan, the elucidation of mechanism of its injury to plant is an important problem to be solved for the protection of agricultural environment. Therefore, the distribution and the reaction of sulfur dioxide after absorption by plants were experimented with fumigation chambrs. The results obtained are summarized as follows:

1. The greater part of sulfur absorbed as sulfur dioxide from the stomata on leaf surface was easily soluble in water. Accordingly, the determination of water-soluble sulfur was more reliable than that of the total, and the former value had higher correlation with the grade of air pollution. And it was presumed that water-soluble sulfur compounds originated from sulfur dioxide were mainly sulfuric acid with some sulfonic acid, both of them react as harmful substances to plant tissues.
2. The distribution of sulfur as air-pollutant was confirmed with autoradiogram of ³⁵S to be rich in growing leaves, especially in the parts enclosed between their veins, and to be ranged in the following order:
   
                       　　　　 leafblade>leafstalk>stem>root
    
3. Incorporation of sulfur dioxide to protein composed of cystine or methionine was less that of nutritive sulfate through roots. The assimilation of sulfur as sulfur dioxide was not much, ranging 1.9 to 11.5 per cent in leaves of a few crops after fumigation for several days.
4. The fractionation of water-soluble sulfur compounds or the elucidation of such a harmful organic compound should be investigated further.