The effects of Hematoporphyrin Derivative Photoradiation Therapy (HpD-PRT) on Greene's amelanotic melanoma implanted into the anterior chamber of rabbits have been examined by biomicroscopy, fluorescein angiography and histopathology. The tumors were irradiated 24 hours after injection of HpD when both the porphyrin concentration and the porphyrin ratio tumor/iris were highest. Blanching and shrinkage of tumors were the first signs of tumor destruction. Fluorescein angiography as soon as 20 minutes after irradiation found non-perfusion of blood vessels at the tumor surface. Histopathological observation of vessel wall destruction is in agreement with this finding. Subtotal tumor necrosis was demonstrated in 12 out of 13 experiments. Necrosis was complete in only one experiment. Clusters of viable tumor cells were found when shielded behind pigment, at the tumor periphery and around some blood vessels. Lens damage was observed after irradiation when the iris pigment epithelium was disorganized by the tumor. The iris contained high concentrations of porphyrin and PRT resulted in depigmentation, non-perfusion of the capillary bed, damage to larger iris vessels and finally atrophy. Light intensity measurements were performed in vivo during PRT. The average effective attenuation coefficient at 630 nm was 0.56 mm-1 at the beginning of irradiation and 0.87 nm-1 at the end. Results indicate that as a treatment HpD-PRT in itself might be insufficient but may prove to be an effective modality in combination with other tumor destructive therapies.