The effects of aphidicolin, a specific inhibitor of DNA alpha-polymerase, have been studied on various cellular end-points and on DNA strand break repair. In the concentration range 0.02-2 micrograms/ml DNA synthesis was strongly inhibited resulting in a concomittant loss of cell proliferation ability; RNA and protein synthesis were unaffected in this range. At these concentrations PLD repair in X-irradiated plateau-phase cells was unaffected even after 7 hours treatment with aphidicolin; however, at higher concentrations (greater than 2 micrograms/ml) PLD repair was inhibited. We show that in the low concentration range (less than 2 micrograms/ml) PLD repair can be seen in exponentially growing cells and from experiments with synchronized cells we establish that the PLD repair observed can be attributed to the S-phase population, the survival of G1-cells not being affected by aphidicolin. The 'promotion' of PLD repair in exponentially growing cells was in excess of that observed for the same cells in balanced salt solution in which PLD repair is usually observed. At high concentrations (greater than 2 micrograms/ml) of aphidicolin, both X-irradiated and control S-cells were killed increasingly as concentration increased. The repair of DNA strand breaks (single and double) was unaffected in the low concentration range, but a strong inhibition was observed at high concentration. It is concluded from these results that alpha-polymerase, which is strongly inhibited at low concentrations of aphidicolin as evidenced by the inhibition of DNA synthesis, plays no major part in the repair of DNA strand breaks or in the repair of PLD.