Two plant growth regulators, kinctin and 2,4-D, were tes ted on soybean plants to find their relative effects on nitrogen fixation. 2,4-D gave much more severe effects on both morphologically and physiologically than did the kinciin. As a result of the treatments with exogenous kinetin and 2,4-Dt the leaf-area, leaf-number, and the height of the plants was reduced. Leaves showed chlorosis, and the chlorophlls in the leaves (mostly the chlorophyll b) was markedly decreased，the accumulation of the dry matter in the tissue fractions diminished, and other apparent changes involved the shoot-root ratio and the chlorophyll-a to chlorophyll-b ratio. The nitrogen fixing (acetylene reduction) activity of intact plants or of detached nodules was suppressed by kinetin and 2,4-D. One of the detrimental effects of 2,4-D or kinetin on nitrogen fixation was a decrease in the root-nodulc-number. It is presumed that the nodule-initiation or nodulc-dcgcncration is inhibited or promoted by the stimulation with kinctin or 2,4-D.