Cesium-137 is a radionuclide of concern in fallout from reactor accidents or nuclear detonations. When ingested or inhaled it can expose the entire body for an extended period of time potentially contributing to serious health consequences ranging from acute radiation syndrome to increased cancer risks. In order to identify changes in gene expression that may be informative for detecting such exposure and to begin examining the molecular responses involved we have profiled global gene expression in mice injected with 137CsCl. We extracted RNA from the blood of control or 137CsCl-injected mice at 2 3 5 20 or 30 days after exposure. Gene expression was measured using Agilent Whole Mouse Genome Microarrays and the data was analyzed using BRB-ArrayTools. Three-month old male C57Bl/6 mice were injected intraperitoneally with 8.0 xc2 xb1 0.3 MBq 137CsCl solution in a volume of 50 xce xbcL or left as controls. Groups of treated and control mice were sacrificed at intervals during the first 2-30 days after exposure and total blood was collected using cardiac puncture. RNA was extracted from the blood globin-transcript reduced and subjected to whole genome expression microarray analysis.