Membrane-type metalloproteinase-I (MTI-MMP) is a transmembrane metalloproteinase, which activates pro-gelatinase A. There has been disagreement as to whether the cell types expressing MTI-MMP are cancer cells or stromal fibroblasts. Using human gastrointestinal carcinomas, the present study disclosed the tissue localization of MTI-MMP mRNA by in situ hybridization and ultrastructural localization of its protein by immunoelectron microscopy. In normal colon and stomach tissues, MTI-MMP mRNA and protein were negative or faintly positive both in epithelial cells and in stromal fibroblasts, except in the fundic gland of the stomach, which showed the positivity for MTI-MMP. In contrast, gastrointestinal cancer tissue showed over-expression of MTI-MMP mRNA and protein both in cancer cells and in stromal cells (fibroblasts). Stromal fibroblasts also expressed mRNA for gelatinase A and type-I procollagen. Double immunohistochemistry revealed that macrophages were also positive for MTI-MMP. Immunoelectron microscopy showed that MTI-MMP was localized along the plasma membrane of cancer cells and macrophages and in rough endoplasmic reticulum of fibroblasts. The present study reveals a dual over-expression pattern of MTI-MMP both in cancer cells and in stromal fibroblasts; the expression in cancer cells may be related to the invasive growth, whereas that in fibroblasts may be related to the tissue remodeling process caused by invasive growth of cancer cells.