While photodynamic therapy (PDT) has been recognized as a promising therapeutic modality for the treatment of various cancers and diseases, developments of effective photosensitizers are highly desired to improve the prospect for the use of PDT. In this study, we evaluated DH-II-24, a new photosensitizer, for antitumor PDT in vitro and in vivo. Loaded into human colorectal carcinoma cells (HCT116), DH-II-24 was primarily accumulated in mitochondria, lysosomes, and endoplasmic reticula. Administration of DH-II-24 followed by light exposure induced necrotic cell death in a dose-dependent manner, whereas DH-II-24 in the absence of light induced minimal cell death. In order to investigate the distribution and phamacokinetics of the photosensitizer in vivo, DH-II-24 was intravenously injected to female BALB/c nude mice. Fluorescence imaging in vivo showed that DH-II-24 was rapidly distributed across the entire body and then mostly eliminated at 24 h. Next, effectiveness of DH-II-24-mediated PDT was examined on colorectal carcinoma xenografts established subcutaneously in BALB/c nude mice. DH-II-24 (1 mg/kg, i.v. administration) followed by light exposure significantly suppressed growth of xenograft tumors, compared to light exposure or DH-II-24 alone. Histological examination revealed necrotic damage in PDT-treated tumors, concomitantly with severe damage of tumor vasculature. These results suggest that DH-II-24 is a potential photosensitizer of photodynamic therapy for cancer. (Cancer Sci 2009; 100: 2431–2436)