The ability of synchronized Ehrlich ascites tumour cells to repair PLD was measured by introducing delays in their progression through the cell cycle either in the same phase as that where the irradiation was given or in a subsequent phase. Cells were incubated for this purpose either in balanced salt solution which nonspecifically delayed progression in all cell cycle phases or with 0.5 µg/ml aphidicolin which delayed cells in S-phase. Cells which had been delayed in their progression through the cell cycle were able to repair PLD irrespective of the phase at which they were held. In cases where the delay in the progression through the cell cycle was introduced in a phase subsequent to that of the exposure to irradiation, repair of PLD was observed only if the cells had not passed the G1/S-border or mitosis. Based on these results, the importance of G1/S-border and mitosis in the fixation of PLD is suggested.