Members of the Bcl-2 protein family that share only the Bcl-2 homology 3 (BH3) domain are known mostly as sentinels for apoptotic stimuli and initiators of apoptosis. One BH3-only protein, Bim, is the major physiological antagonist of the prosurvival proteins in B and T lymphocytes. It is required for hematopoietic homeostasis and to preclude autoimmunity. Here, we show that the Bim_EL isoform, which was predominant in T cells, existed in both phosphorylated and unphosphorylated forms. Whereas the unphosphorylated Bim_EL was sequestered to microtubules by means of a direct interaction with tubulin, the phosphorylated protein was released from microtubules. The freed Bim_EL was subjected to caspase cleavage at an early stage of apoptosis induced by stimuli that activate either the mitochondria- or death receptor-dependent apoptosis pathway. The N-terminally cleaved Bim_EL became hyperactive in inducing apoptosis because of its more efficient targeting of Bcl-2. Thus, unlike many other BH3-only proteins, Bim_EL can be activated downstream of the caspase cascade, leading to a positive feedback amplification of apoptotic signals.