In vitro experiments were performed to evaluate the direct cytotoxic and photosensitizing effects of dihematoporphyrin-ether (DHE) on the head and neck squamous cancer cell line, UM-SCC-38. Normal fibroblasts, normal cultured keratinocytes, and the UM-MEL-1 pigmented malignant melanoma cell line were used as controls. The parameters of duration of light exposure, drug concentration, and incubation periods were studied. Uptake of DHE by squamous cancer cells, as assessed microscopically by intensity of fluorescence, was rapid and reached a dose-dependent maximum intensity within 10 minutes. Cells were irradiated with polychromatic light with an energy of one milliwatt/cm2 at a distance of 1.5 ft. Cells growing in plastic dishes were incubated in the dark for 1 to 4 hours with DHE in concentrations that ranged from 1.25 to 50 micrograms/ml. The cell monolayers were washed and irradiated for periods of time ranging from 15 to 120 min. Dose-dependent loss of cell viability could be detected by trypan blue dye uptake as early as 1 hour after radiation and continued to increase until 4 hours after light exposure. No further loss of cell viability was observed over the next 10 hours. There was no phototoxicity in the absence of DHE. UM-SCC-38 cells were more sensitive to the photosensitizing effects of DHE than were either normal fibroblasts or malignant melanoma cells. Recovery from the photosensitizing effects of DHE was observed if the DHE-containing medium was removed and the cells were incubated in the dark for periods that ranged from 1 to 14 hours before light exposure. UM-SCC-38 cells recovered more rapidly than normal fibroblasts, normal keratinocytes, or UM-MEL-1 cells.