We have measured DNA strand breaks induced by ionising radiation in nucleated cells from freshly isolated whole blood from normal human subjects. Samples were taken after subjects had fasted overnight and again 1 h after they had eaten breakfast in combination with approximately 35 mg/kg vitamin C. Damage was measured by single cell gel electrophoresis (the 'comet' assay), in which DNA single strand breaks generate a comet tail streaming from the nucleus. In repeat experiments on 6 subjects a reduction in DNA damage, as indicated by a highly significant decrease in overall comet length, was observed following vitamin C ingestion, both in the unirradiated control blood samples and in the dose response to ionising radiation damage. In addition, consistent differences in dose response between individual subjects were found. The peak effect was 4 h after intake of food and vitamin C. An effect was also seen with vitamin C alone and after breakfast without additional vitamin C. Protection against strand breakage was also seen in Ficoll-separated mononuclear cells but evidence was not obtained for protection of separated, mitogen stimulated T-lymphocytes either against ionising radiation cell killing in a clonal assay, or against clastogenicity assessed by micronucleus formation following one cell division. Exposure of separated lymphocytes in vitro to vitamin C, at doses greater than 200 microM, did not offer protection but induced strand breakage. Our results raise the possibility that variation in normal diet may not only affect susceptibility to endogenous oxidative damage, but may affect some responses of the individual to radiation.