Cells from the established human line NHIK 3025 were labelled with hematoporphyrin derivative in vitro. Subsequently, the cells were treated with light and hyperthermia. The cells could be irradiated either before, during or after the incubation at a hyperthermic temperature. It was shown that hyperthermia given shortly after the light exposure gave a synergistic killing effect. In spite of some loss of porphyrins from the cells, the light sensitivity increased 20 min after a light irradiation. At later times, the cells apparently repaired some of the photodynamic damage at 37 degrees C. At higher temperatures, the repair was inhibited.