Abstract
The origin of the 2-phase (cap, no cap) macroscopic kinetic model of the end of a microtubule is reviewed. The model is then applied to a new theoretical problem, namely, the Mitchison-Kirschner experiment in which aggregated microtubules in solution spontaneously decrease in number (shorten to disappearance) while the surviving microtubules increase in length. The model fits the experiments without difficulty.