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With 7 figures in the text) SUMMARY The movement of radioactivity added in the form of indoleacetic acid (as the natural hormone) or as 2,4-dichlorophenoxyacetic acid (as a synthetic growth regulator) was followed in segments cut from petioles of Phaseolus vulgaris.The movement of i-^C added as IAA was strongly basipetally polar, although significant amounts moved acropetally during 24 hours. The velocity of basipetal movement was 6 mm/hour for IAA-[i4C], and was the same for donor concentrations of 5 or 50 pM. The flux increased with increasing concentration in the donor blocks, but a progressively smaller percentage of the applied radioactivity was transported.2,4-D-[i4C] was found to have very similar transport properties to those of IAA-P^C], except that its velocity of basipetal movement was much less, namely i mm/hour. After correction had been made for the much slower net loss of 2,4-D from its donor blocks, the acropetal movement of the i^C from the two substances was essentially the same.

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