Amount of ATP in HeLa cells in various phases of growth was measured after He-Ne laser irradiation (100 J/m2, 10 W/m2, 10 s) by a bioluminescent luciferin-luciferase method. In cells of the exponential phase of growth, the amount of ATP (basal level 8 x 10(-16) mole/cell) starts to increase in 15 min after the irradiation with a maximum (170% above the basal level) at 20 min, and then decreases gradually to the basal level. The sensitivity of the cells to He-Ne laser radiation is lowest in lag-phase of growth, then increases to a plateau (approximately 185% above the control level) from 5th day of cultivation.