We have used an indirect method to compare the dynamic properties of microtubules (MTs) in the main shaft and distal regions of the axon. Individual MTs are staggered along the length of the axon and consist of a labile domain situated at the plus end of a stable domain (Baas and Black, 1990). As a result of this organization and the plus-end- distal orientation of axonal MTs, the most distal region of the axon consists entirely of labile domains, while the main shaft consists of a mixture of labile and stable domains. In this study, we wished to determine whether the labile domains extending into the distal axon differ in their dynamic properties from the labile domains terminating in the main shaft. To address this issue, we used immunoelectron microscopy to compare the tyrosination state of the labile domains terminating in these 2 axon regions. Because detyrosination is a polymerspecific modification of alpha-tubulin that accumulates with time, the levels of tyrosinated alpha-tubulin will be a reflection of the age, and hence dynamic properties, of the polymer. To maximize our chances of visualizing potential differences, we varied the concentration of the primary antibody in these experiments. Our studies indicate that the stable domains are generally deficient in tyrosinated alpha-tubulin, while the labile domains contain clearly detectable levels. Within the labile domain, the subsection closer to the plus end of the MT contains relatively higher levels of tyrosinated alpha- tubulin than does the subsection farther from the plus end, suggesting that the levels of tyrosinated alpha-tubulin in the labile domain may gradually increase as one moves away from the stable domain toward the plus end of the MT. Although these observations apply to the labile domains in both regions of the axon, the labile domains extending into the distal region contain comparatively higher levels of tyrosinated alpha-tubulin than do the labile domains terminating in the main shaft. These results are consistent with the view that highly dynamic MT polymer is present throughout the axon, but that the polymer nearest the advancing growth cone is particularly dynamic.