The effect of the DNA polymerase inhibitors adenine 9-beta-arabinofuranoside (ara-A), cytosine 1-beta-arabinofuranoside (ara-C), and aphidicolin on X-radiation sensitivity was studied in a group of exponentially growing squamous cell carcinoma cell lines. The tumour cell lines varied in radiation sensitivity, with D0 (radiation sensitivity) values ranging from 1.0 to 3.9 Gy. The addition of non-toxic concentrations of ara-A 30 min before irradiation and removal 30 min after irradiation potentiated cell killing in five of eight cell lines. Four of these five responsive cell lines were relatively radioresistant lines, having D0 > 2.0 Gy. One of the cell lines was more radiosensitive (D0 = 1.4 Gy). Ara-A was also effective in potentiating killing in the radioresistant cell lines even when added 60 min after irradiation. Pre- or post-treatment with ara-A had no effect on X-ray sensitivity of the other three relatively sensitive cell lines (D0 ranging from 1.0 to 1.3 Gy). Both ara-C and aphidicolin were effective in potentiating X-ray sensitivity in JSQ-3, a relatively resistant cell line that was sensitized by ara-A treatment, but they had no effect on the X-ray sensitivity of SCC-61, a relatively radiosensitive cell line that was insensitive to ara-A effects on X-ray response. At the concentrations used, the polymerase inhibitors were equally effective in inhibiting DNA synthesis.