This work deals with measurements of the transport of radio-activity from [(3)H] indole acetic acid during early stages of auxin-induced vascular differentiation. Transverse wounds were made in stems of Phaseolus vulgaris L. and the radioactivity which was transported around these wounds was measured. Treatments with unlabelled auxin above the wounds increased transport within 16 hours of the time they were first applied while the maturation of vascular elements in response to these treatments started only on the 3rd day. It is concluded that increased transport is an early stage of vascular differentiation, and this phenomenon can account for the organized differentiation of vascular elements in vertical rows and defined bundles.