AbstractMitotic spindles isolated from sea urchin eggs can be reactivated to undergo mitotic processes in vitro. Spindles incubated in reactivation media containing sea urchin tubulin and nucleotides undergo pole‐pole elongation similar to that observed in living cells during anaphase‐B. The in vitro behavior of spindles isolated during metaphase and anaphase are compared. Both metaphase and anaphase spindles undergo pole‐pole elongation with similar rates, but only in the presence of added tubulin. In contrast, metaphase but not anaphase spindles increase chromosome‐pole distance in the presence of exogenous tubulin, suggesting that in vitro, tubulin can be incorporated at the kinetochores of metaphase but not anaphase chromosomes. The rate of spindle elongation, ultimate length achieved, and the increase in chromosome‐pole distance for isolated metaphase spindles is related to the concentration of available tubulin. Pole‐pole elongation and chromosome‐pole elongation does not require added adenosine triphosphate (ATP). Guanosine triphosphate (GTP) will support all activities observed. Thus, the force generation mechanism for anaphase‐B in isolated sea urchin spindles is independent of added ATP, but dependent on the availability of tubulin. These results support the hypothesis that the mechanism of force generation for anaphase‐B is linked to the incorporation of tubulin into the mitotic apparatus. (If, in addition, a microtubule‐dependent motor‐protein(s) is acting to generate force, it does not appear to be dependant on ATP as the exclusive energy source).