Rhodamine 123 (Rh 123), a mitochondrial-specific dye with an absorption maxima at 511 nm, was tested as a new chemosensitizing agent for argon laser treatment of P3 human squamous carcinoma cells. After exposure of P3 cells in vitro to Rh 123 at doses of 1, 3, 6, and 10 micrograms/mL for one hour, we observed significant inhibition of DNA synthesis, except at the lowest dose. Rhodamine 123 at 1 microgram/mL was selected to sensitize P3 tumor cells for treatment with the monochromatic argon laser at 514.5 nm. Exposure of P3 cells to laser energy levels of 700 to 950 J/cm2 (36 degrees C to 40 degrees C) after sensitization with Rh 123 completely inhibited tumor development of the P3 cells transplanted subcutaneously into nude mice. Control P3 cells treated with the laser alone at 36 degrees C to 40 degrees C or only with Rh 123 formed visible tumors by one week and continued to grow for the entire-week observation period. These results show that Rh 123 is a highly sensitive new fluorochrome for argon laser phototherapy of human squamous carcinoma cells.