Experiment was designed to clarify the effects of near-UV laser irradiation on cultured cells. Mouse Mbroblast cells (3T3 cell) were irradiated by N₂-laser (wavelength: 337nm, pulse width: 10ns, 40Hz). This cells showed a decrease in cell number after treatment, but growth rates were found to be equivalent to that of control cell population. The survival curve of this cells suggested that this cell-line were reacting to a multihit or multitarget phenomenon for near UV-laser radiation. When this cells treated by vitamin A, the effects of UV-laser radiation was remarkably enhanced.