The availability of a cell-division-cycle (cdc) mutant in the fission yeast S. pombe, wee 1-50, has made possible the production of a large population of G1 nuclear-stage synchronized cells. During their development, yeast cells from the G1 into the G2 nuclear stages were treated with X-rays and UV radiation at various doses. The DNA pre-replicative and replicative phases were the most sensitive to both cell lethality and mutant induction with either X-rays or UV radiation. The trends of induced biological effects that were observed suggest that the induction of mutations is dependent on the number of unrepaired DNA lesions that reach the replicating fork or of those that occur at that time. The X-ray-induced mutations were earlier saturated, possibly because of the higher number of lethal lesions so induced.