The effect of 5-fluorouracil (5-FU) and uracil starvation on the UV-sensitivity ofEscherichia coli 15 was followed in a 60 minutes period after the irradiation. The presence of 5-FU caused a significant increase of radioresistance when compared with the control, on the other hand under conditions of uracil starvation the cells were slightly more sensitive than in the control sample. The DNA synthesis was highly inhibited under conditions of uracil starvation, but the same effect was observed in the presence of 5-FU. Maximum inhibition of protein synthesis was observed also when uracil was omitted from the cultivation medium and approximately 50% of inhibition was noticed in the sample with 5-FU. The increase of resistance caused by the presence of 5-FU in the 60 minutes period after UV-irradiation cannot be explained by prolonging the sensitive postirradiation phase. It seems probable that in the presence of 5-FU the conditions are favourable for a better course of the repair processes.