Auxin controls the orientation of cortical microtubules in maize coleoptile segments. We used tyrosinylated α-tubulin as a marker to assess auxin-dependent changes in microtubule turnover. Auxin-induced tyrosinylated α-tubulin, correlated with an elevated sensitivity of growth to antimicrotubular compounds such as ethyl-N-phenylcarbamate (EPC). We determined the affinity of α-tubulin to EPC and found that it was dramatically increased when the tubulin was de-tyrosinylated. By proteolytic cleavage of the carboxy terminal tyrosine, such an increased affinity could be induced in vitro. Thus, the auxin-induced sensitivity of growth to EPC is not caused by an increased affinity for this inhibitor, but caused by a reduced microtubule turnover. Double visualization assays revealed that the transverse microtubules induced by auxin consist predominantly of tyrosinylated α-tubulin, whereas the longitudinal microtubules induced by auxin depletion contain de-tyrosinylated α-tubulin. The results are discussed in terms of direction-dependent differences in the lifetime of microtubules.