Our approach to examine the mechanism(s) of action for photodynamic therapy (PDT) has been via the generation of PDT-resistant cell lines. In this study we used three human cell lines, namely, human colon adenocarcinoma (HT29), human bladder carcinoma and human neuroblastoma. The three photosensitizers used were Photofrin, Nile Blue A and aluminum phthalocyanine tetrasulfonate. The protocol for inducing resistance consisted of repeated in vitro photodynamic treatments with a photosensitizer to the 1–10%-survival level followed by regrowth of single surviving colonies. Varying degrees of resistance were observed. The three induced variants of the HT29 cell line were the most extensively studied. Their ratios of increased survival at the LD90 level range between 1.5- and 2.62-fold more resistant.