Photodynamic effect of HPD plus light on human stomach cancer cells (MGC-803) in the mitotic phase and interphase was studied. The synchronized mitotic cells were selected from the monolayer of asynchronous growing cells by shaking the culture flask. It was found that the killing frequency of HPD photosensitization was markedly different in the mitotic phase and interphase of synchronous growing MGC-803 cells, but a very small difference was found in HPD alone or light alone groups. The results show that, for adhesive cells in the two phases, there was a marked difference in damage pattern and recovery rate of mitosis 36 hr, 60 hr and 108 hr after photoradiation. The tolerance of mitotic cells to HPD plus light was much greater than that of the interphase cells. In the interphase, a great many of nude nuclei were seen and in the mitotic phase, cytokinesis was arrested resulting in many binucleated or multinucleated cells.