Photochemical internalisation (PCI) is a method for efficient delivery of macromolecules, like genes used in gene therapy, to the cytosol of cells. Most macromolecules enter a cell by endocytosis and are trapped in the endocytic vesicles. PCI is based on photochemical treatment disrupting the endocytic vesicles, i.e. a photosensitiser localised in the membranes of endocytic vesicles ruptures these vesicles upon exposure to light and endocytosed macromolecules are released into the cytosol. Thus, PCI leads to an enhanced biological effect mediated by the liberated macromolecules. In the present work, this method was employed together with the non-viral vector polyethyleneimine (PEI) for delivery of the pro-apoptotic gene TNF-related apoptosis inducing ligand (TRAIL). The aim was to evaluate the potential of PCI-mediated delivery of the PEI/TRAIL complexes to enhance cancer cell death in vitro. Furthermore, the apoptotic death pathway following PCI of the TRAIL gene in colon carcinoma HCT-116 cells was investigated. The PCI-mediated effect was compared to the effects induced by the photochemical treatment alone or the treatment with the TRIAL gene alone. The effect of PCI was tested with respect to TRAIL expression, induction of apoptosis and cell survival using various methods from the fields of biochemistry and cell biology. In addition, the PCI effect on the level of several key molecules involved in apoptosis was investigated using the Western blotting technique. The results showed that PCI enhanced the expression and the cytotoxic effect of the TRAIL gene, and it also enhanced the induction of apoptosis, as compared to the other two treatments. It was shown that in PCI, the TRAIL gene and the photochemical treatment acted additively rather than synergistically. Although the apoptotic mechanisms behind PCI of the TRAIL gene was not fully elucidated, there were indications that in HCT-116 cells apoptosis proceeds through the extrinsic (death receptor mediated) pathway, however without activation of the main executioner caspase, caspase-3. Moreover, photochemical up-regulation of the TRAIL death receptor was detected.