In this study we assessed cell response to ionizing radiation doses in the diagnostic X-ray range, using metabolic performance as the biological end point. HeLa cells were irradiated between 3.5 mGy (standard deviation (SD=2.5%) and 55.1 mGy (SD=0.9%) with 110 kV X-rays, and after 7 days metabolic capacity of cultures was measured spectrophotometrically with resazurin. Cells receiving 7.5 mGy or greater showed impaired proliferation. At 7.5 mGy this effect was prevented by nitric oxide synthase inhibitor N G-monomethyl-L-arginine but not by ascorbic acid or p38 MAPK inhibitor SB203580. We conclude that HeLa cell proliferation is adversely affected by doses within the range of some radiological examinations, through a mechanism involving nitric oxide.