This investigation was undertaken to study the influence of 2.4-D on the tuber formation of the excised sprout tips of potato cultured on the nutrient medium in vitro. According to the results obtained, a remarkable increase of tuber formation of the potato due to the application of 2.4-D was recognized. At the same time, the accumulation of starch in the sprout tips cultured in the presence of 2.4-D increases distinctly keeping pace with the decrease of reducing sugar content. While the sprout treated with 2.4-D showed no significant variation in auxin level of the sprout, in spite of a slight loss of free auxin content. The experimental data mentioned above may be correlated with the onset of the tuberization. Further explanation on this phase of the decrease in free auxin content remains to be investigated.