We have studied the interaction of preformed microtubules (MTs) with the kinetochores of isolated chromosomes. This reaction, which we call MT capture, results in MTs becoming tightly bound to the kinetochore, with their ends capped against depolymerization. These observations, combined with MT dynamic instability, suggest a model for spindle morphogenesis. In addition, ATP appears to mobilize dynamic processes at captured MT ends. We used biotin-labeled MT seeds to follow assembly dynamics at the kinetochore. In the presence of ATP and unlabeled tubulin, labeled MT segments translocate away from the kinetochore by polymerization of subunits at the attached end. We have termed this reaction proximal assembly. Further studies demonstrated that translocation could be uncoupled from MT assembly. We suggest that the kinetochore contains an ATPase activity that walks along the MT lattice toward the plus end. This activity may be responsible for the movement of chromosomes away from the pole in prometaphase.