We examined the modulation of gene expression in Syrian hamster embryo (SHE) cells at various times following exposure to low doses of ionizing radiation. Early passage SHE cells were irradiated in plateau phase (greater than 95% G0-G1 cells) with 21-cGy fission-spectrum neutrons, 75-cGy X-rays, or 90-cGy gamma-rays, none of which induced more than 10% loss in cell viability. RNA harvested at various times after exposure was examined for levels of particular RNA species by dot blot and Northern blot hybridizations. Levels of beta-actin-specific RNA decreased within 15 min after exposure of the cells. The kinetics of repression of beta-actin mRNA were similar for all qualities of radiation (X-rays, gamma-rays, and neutrons) for 12 h post-irradiation. Within 1 h after neutron exposure (21 cGy), we observed a decrease in accumulation of RNA species (relative to RNA from nonirradiated cells) encoding the enzyme ornithine decarboxylase; this decrease continued for up to 12 h. Similar results were obtained with gamma- and X-rays. RNA encoding interleukin 1, however, was induced by 3 h after neutron irradiation but reduced to background levels by 7 h. Amounts of rRNA remained constant in all experiments, although total transcription on a per cell basis was reduced within 15 min following irradiation and did not return to normal until 7 h post-irradiation. No alterations, relative to untreated control cells, in overall cell viability or the rate of cell cycle progression were observed in cells either immediately or within 24 h post-irradiation. Our results demonstrate modulation of specific genes following low-dose irradiation. In addition, our findings suggest that some molecular responses to different qualities of ionizing radiation (X-rays, gamma-rays, and neutrons) may be similar.