Following the chemical and photophysical characterization of the AL(DIPY)3 complexes, their photobiological evaluation was assessed in view of their application as PSs in PDT. The AL(DIPY)3 chelates used for the in vitro studies were chosen in terms of their solubility in DMSO to obtain stock solutions with concentrations of 1-2 mM. Therefore, eight of the complexes were investigated with regards to their phototoxic effect against the CT26 mouse colon carcinoma cell line (13a, 13c, 13d, 13f, 13g, 13i, 16, 17). Cell viability was assessed by using resazurin assay where the cell survival is determined in comparison with the non-treated control sample which corresponds to 100% cell viability. In a first set of studies, the toxicity of the selected compounds was evaluated in the absence of light using the CT26 cell line. Most of the complexes showed no toxicity up to 20 μM except for 17 which displayed 40-50% of cell death at 20 μM (Error! Reference source not found.). Therefore, in the following phototoxicity studies, the highest concentration of 17 that was tested was 10 μM.