Axonal microtubules consist of two distinct domains that differ in tyrosinated-tubulin staining. One domain stains weakly for tyrosinated-tubulin, while the other stains strongly, and the transition between these domains is abrupt; the tyrosinated-tubulin-poor domain is at the minus end of the microtubule, and the tyrosinated-tubulin-rich domain extends from the plus end of the tyrosinated-tubulin-poor domain to the end of the microtubule. The tyrosinated-tubulin-poor domain is drug- and cold-stable, whereas the tyrosinated-tubulin-rich domain is drug-labile, but largely cold-stable. STOP (stable-tubule-only-polypeptide) has potent microtubule stabilizing activity, and may contribute to the cold and drug stability of axonal microtubules. To evaluate this possibility, we examined STOP association with the different types of microtubule polymer in cultured sympathetic neurons. By immunofluorescence, STOP is present in the cell body and throughout the axon; axonal staining declines progressively in the distal portion of the axon, and reaches lowest levels in the growth cone. Growth cone microtubules, which are drug and cold labile, do not stain detectably for STOP. To examine individual axonal microtubules for STOP, we used a procedure that causes microtubules to splay out from the main axonal array so that they can be visualized for relatively long distances along their length. Both tyrosinated-tubulin-rich and tyrosinated-tubulin-poor polymer stain for STOP, but STOP is several-fold more concentrated on tyrosinated-tubulin-poor polymer than on tyrosinated-tubulin-rich polymer. These results are consistent with STOP dependent stabilization of axonal microtubules, with the difference between cold-stable polymer versus cold- + drug-stable polymer determined by the amount of STOP on the polymer.