Human gamma-glutamyl hydrolase (hGH) plays an important role in the metabolism of folic acid and the pharmacology of antifolates such as methotrexate. We have previously cloned and characterized hGH cDNA and its gene. We report here that the levels of hGH gene expression are high in tissues such as liver, kidney, and placenta as determined by Northern blot and RT-PCR analyses. In contrast, hGH expression is relatively low in spleen, lung, small intestine, and peripheral blood leukocytes. In addition, high levels of hGH mRNA were detected in most cancer cell lines examined. There was no significant difference in hGH mRNA levels between breast cancer tissues and their normal counterparts, although breast cancer tissues generally appeared to have heterogeneous expression of hGH mRNA. Deletion analysis and transient transfection assays were performed. A positive regulatory element located from -52 to +4 relative to the transcriptional start site was found to be required for basal promoter activity in both HepG2 and MCF-7 cells. Also, transcriptional inhibitory elements were found at -96 to-52 and +88 to+104 in MCF-7 cells, but not in HepG2 cells. These data provide novel insights into the regulation of hGH gene transcription in HepG2 and MCF-7 cells.