PEG intracellularly! Although long known to enhance residence half-life of peptides in serum and lysates, the effect of PEGylation on biological probes in cells has received only limited attention. Here it is shown that phosphoramidate-linked PEGylated proapoptotic peptides display a dramatically increased stability in Jurkat cell lysate and a homogenous intracellular distribution as well as high apoptotic activity after introduction into cells.