['Lysis of endocytic organelles is a necessary step in many cellular delivery methodologies. This is achieved efficiently in the photochemical internalization approach but the cell death that accompanies this process remains a problem.', 'We investigate the mechanisms of cell death that accompanies photochemical internalization of the fluorescent peptide TMR-TAT.', 'TMR-TAT kills cells after endocytosis and light irradiation. The lysis of endocytic organelles by TMR-TAT causes a rapid increase in the concentration of calcium in the cytosol. TMR-TAT co-localizes with endocytic organelles containing calcium prior to irradiation and photochemical internalization leads to the release of the lumenal content of these organelles. Ruthenium red and cyclosporin A, inhibitors of calcium import in mitochondria and of the mitochondria permeability transition pore, inhibit cell death.'] Does tAT-mediated photochemical internalization result in cell killing by causing the release of calcium into the cytosol of cells?