Diesel particulate extract (DPE) is a complex mixture containing polycyclic aromatic hydrocarbons. It is a human health concern and contributor to both ambient and occupational air pollution. In this study, alterations in gene expression in normal human mammary epithelial cells (NHMECs) in response to DPE have been investigated using DNA microarrays. The capacity for DPE to induce cytotoxicity was determined over a concentration range of 0.025 - 4 ul/ml using the MTT (3-[4, 5dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assay. A dose dependent decrease in cell viability was observed with a 50% cell death occurring at a concentration of 1.5 ul/ml. Transcriptional response in NHMECs was monitored after a 24h exposure to non-cytotoxic concentrations of DPE (0.1, 0.2, 0.3 and 0.4 ul/ml) using DNA oligonucleotide microarrays (U133A, Affymetrix). Of the total 22,000 genes present on the gene array, exposure to DPE altered expression of 117 to 493 genes in a dose dependent manner when exposed to different concentrations by more than 2-fold. Several metabolism genes were induced significantly (p < / = 0.05) in a dose dependent manner (e.g. AKR1C2, GLRX, SOLE, INSIG1, NOO1, GPX2), however, others had increased expression but remained similar for all doses (CYP1A1, CYP181, HSD1782). Further, signal transduction genes (NCOA 1, SPRR2A, TNFSF9), immune inflammation response genes (IF144, IL 13RA2, PTGS2) and apoptosis genes (SERPIN82, SPP1, TXNRD1, ALOX158) were all up-regulated significantly compared to the solvent control (DMSO). Cell cycle control genes (CCN81, CCNB2, MAD2L 1, CDC6, TGFB2, CDK2) and DNA replication genes (GMNN, RFC3, RFC4, PCNA) were significantly (p < / = 0.05) down-regulated. These results suggest several genes as potential biomarkers that may be of value for monitoring DPE exposed workers.