['To investigate the effect of low doses of 60 kVp X-rays on in vitro transformation frequency.', 'HeLa x skin fibroblast human hybrid cells were used to assay transformation from the non-tumorigenic to the tumorigenic phenotype. Subconfluent cultures of cells were exposed to a range of doses of 60 kVp X-rays and seeded for assay of transformation after 24 h post-irradiation holding. Experiments were repeated at least three times and the data pooled for analysis. Transformation frequencies were compared with those of sham-irradiated controls.', 'At doses < 1 cGy, the observed transformation frequencies were significantly less than those seen in unirradiated cells.'] Do low doses of diagnostic energy X-rays protect against neoplastic transformation in vitro?