Calcium ion induces rapid depolymerization of microtubules, and the mechanism of interaction of Ca2+ with cytoskeletal components may be metabolically significant. The sufficiency of an endwise depolymerization model for describing calcium-induced disassembly was demonstrated by three criteria: (a) correlation of the observed time course of disassembly with the initial polymer length distribution using a kinetic model described elsewhere (Kristofferson, D., Karr, T. L., and Purich, D. L. (1980) J. Biol. Chem. 255, 8567-8572); (b) observation that the initial rate of calcium-induced disassembly parallels the microtubule number of concentration which is experimentally manipulated by mechanical shearing; and (c) determination of the average polymer length, microtubule number concentration, and extent of depolymerization at various levels of added calcium ion to achieve successive partial disassembly extents. The rate constant for protomer release and other aspects of the disassembly mechanisms are also presented. Critical concentration measurements at low calcium ion levels are also consistent with endwise interactions.