Microtubules in an M-phase cell are much more dynamic, on average, than the microtubules at interphase. Mammalian cells in culture were injected with tubulin that had been covalently linked to a fluorescent dye. After the fluorescent tubulin had become incorporated into the cell's microtubules, an intense laser beam was used to bleach all the fluorescence in a small region. The recovery of fluorescence in the bleached region of microtubules, caused by their replacement by microtubules formed from unbleached fluorescent tubulin from the soluble pool, was then monitored as a function of time. The time for 50% recovery of fluorescence (t_1/2) is thought to be equal to the time required for half of the microtubules in the region to depolymerize and re-form. (Data from W.M. Saxton et al., J. Cell Biol. 99:2175–2187, 1984. © The Rockefeller University Press.)