Photodynamic therapy (PDT) invokes a number of cellular responses. Other studies have shown that PDT induces transcription and translation of heat shock proteins (Hsps). The expression of mitochondrial heat shock protein, Hsp60, was measured following in vitro Photofrin-mediated PDT in the colon cancer cell line HT29 and its PDT-induced resistant variant HT29-P14 as well as the radiation-induced fibrosarcoma cells RIF-1 and its PDT-induced resistant variant, RIF-8A. Basal levels of Hsp60 were found to be similar in the two murine cell lines. In the human model, the resistant HT29-P14 cell line showed a small increase in basal levels relative to its parental population. Incubation with Photofrin (PII) alone or photosensitization caused a significant increase in Hsp60 levels in all cell lines as determined by flow cytometry. A dose-dependent and temporal relationship for PDT response was observed, maximum levels were detected 6-8 h post PDT, at which time, Hsp60 induction was found to be significantly greater in the two resistant variants. Induction in the RIF cells was also found to be greater after incubation with PII alone at the highest doses tested. These results indicate that the presence of PII and the subsequent oxidative stress of PDT can induce Hsp60 and implicated it as a common factor that may contribute to the resistance observed in the induced resistant cells.