Assaying microtubule dynamics in vitro requires stabilized nucleation centers, a method to immobilize individual microtubules onto a surface, and a specialized microscope to image the microtubule. Microtubules are polar structures with different dynamic properties at the plus and minus ends. However, the dynamics of the two ends can be modified by the addition of other proteins, such as microtubule plus-end-tracking proteins (+TIPs), so that it becomes impossible to distinguish the microtubule polarity by measuring the differences in the dynamic properties of the ends alone. In this chapter, we describe a method for labeling tubulin protein with N -hydroxysuccinimide ester fluorescent dyes, enabling the formation of dual-color polarity-marked stable microtubule seeds that can be immobilized onto a microscopic cover glass for imaging by fluorescence microscopy. These seeds create functional nucleation centers for the growth of dynamic microtubules.