Brief intervals of strong flow stretch chains in a semicrystalline polymer melt to form flow-induced precursors, which accelerate crystallization kinetics and transform the morphology. Using commercial isotactic polypropylene, the persistence and lifetime of flow-induced precursors were investigated, focusing on the effects of specific work and annealing time and temperature. Precursors were formed by shearing polypropylene in a rotational rheometer and then quenching to a desired temperature. The crystallization time and crystallization temperature of the sheared samples were investigated using novel rheology and DSC experiments. For sufficiently large shear rates, the appearance of flow-induced precursors is controlled by the applied work. A qualitative change in many properties related to precursors takes place at a critical work value W-c, about 7 MPa for our iPP material. To erase the persistent "memory" of flow-induced nuclei, samples must be annealed for a very long time below 210 degrees C (>3000 mm) or a shorter time above 210 degrees C (e.g., 300 min at 250 degrees C). Annealing above the Hoffman-Weeks temperature (208 degrees C) evidently erases flow-induced precursors much faster, although at 210 degrees C still requiring times a factor of SO longer than the reptation times of the longest chains.