Photodynamic sensitizers are light-absorbing chemicals that cause photoreactions in biologic systems when exposed to light of the proper wave-length. Dihematoporphyrin ethers (DHE) are the active porphyrin derivatives most commonly used as a photosensitizer (Photofrin, QLT). DHE accumulates in tumor tissue and also fluoresces when light activated. A more reliable and less costly screening method for early detection and treatment of colon cancer is needed. The present study was designed to induce adenocarcinoma of the colon in rats with 1,2 dimethylhydrazine (DMH) and attempt to identify tumors early in their evolution by DHE fluorescence. Forty rats were injected with 20 mg/kg of DMH at weekly intervals until sacrifice. Photofrin (3 mg/kg) was injected through the tail vein in each prior to sacrifice. Eight colonic specimens contained invasive adenocarcinoma, seven of which fluoresced when exposed to a Woods lamp. Carcinoma in situ was identified in two specimens by fluorescence, and one fluorescent specimen contained dysplasia.