UV radiation is known to be a potent agent for the induction of programmed cell death (apoptosis) in human skin. However, the mechanistic aspects of UV-induced apoptosis remain ill-defined. In this study the effects of varying periods of UV-irradiation on the human leukaemia HL-60 cell line and on five other human cell lines were investigated. HL-60 cells were found to rapidly undergo apoptosis en masse after short periods of UV-irradiation, whereas prolonged exposure of these cells to this form of radiation induced a more rapid form of cell death which was suggestive of necrosis, the pathological mode of cell death. Similar effects were observed on the U937 (myelomonocytic), Molt-4 (T-lymphoblastoid), and Molt-3 (T-lymphoblastoid) cell lines, whereas the K562 (pre-erythroid) and Daudi (B-lymphoblastoid) cell lines proved to be relatively resistant to the death-inducing properties of UV-irradiation by comparison. UV-induced apoptosis in cell lines was characterized by morphological changes as well as DNA fragmentation into unit multiples of approximately 200 bp, which was indicative of endogenous endonuclease activation. This DNA fragmentation pattern was not detected in cells immediately after UV-irradiation, and was therefore not the result of direct UV-induced DNA damage. UV-induced apoptosis of the HL-60 cell line was found to require extracellular calcium and to be inhibited in a dose-dependent way by zinc added to the culture medium.