Laser photodynamic therapy of superficial malignancies is a promising new approach that will become clinically useful when fluorochromes with high tumor specificity and low toxicity to normal tissues are identified. We recently reported that the mitochondrial dye, Rhodamine-123 (Rh-123), at nontoxic doses, is an effective sensitizing agent for argon laser treatment of human squamous carcinoma and melanoma cells in vitro. We now report the complete inhibition of in vivo tumor development by human M24 melanoma cells transplanted subcutaneously into nu/nu mice after exposure to 1 microgram/ml of Rh-123 for 1 hour and treatment with an argon laser at nonthermal temperatures of 36 to 40 degrees C. Significant in vivo growth was observed for all control tumors. These results demonstrate that Rh-123 enhances the tumoricidal effects of the argon laser at nonthermal temperatures and provides evidence that effective photodynamic therapy may be possible in vivo with the new fluorochrome Rhodamine-123.