A method is disclosed for detecting exposure of organisms to biologically significant or hazardous amounts of ionizing radiation. The method uses nucleic acid microarray hybridization to evaluate biological effects, such as patterns of expression of genes after radiation exposure. Numerous genes are provided which have been found to be responsive to radiation exposure in a variety of cell lines. These genes are incorporated into probe sets, which are exposed to a labeled nucleic acid composition from a test cell, such as cDNA reverse transcribed from mRNA in the test cell, which specifically hybridizes to members of the probe set when the cell has been exposed to a biologically significant amount of ionizing radiation. Whether the nucleic acid composition hybridizes to the nucleic acid molecules representing genes that are differentially expressed is determined. The invention also includes methods for determining a dose response relationship between radiation exposure and differential expression of one or more genes, for example to determine a probable radiation dose in cells that have actually or potentially been exposed to the ionizing radiation. The invention also includes probe sets and microarrays used in this method.