The role of protein phosphorylation in UV induced apoptosis in cultured human preerythrocyte leukemia cells was examined. It showed that the UV induced cell death is occurred by apoptosis. Cells with apoptotic body were observed by UV treatment. In addition to the morphological changes, UV treatment caused a rapid increase of cyto-plasmic Ca++ concentration followed by DNA fragmentation. The increased digestion of DNA in apoptosis process is believed to be the results of increased nuclear endonuclease of which the activity is regulated by the protein phosphorylation.