The matrix-degrading enzyme family of matrix-metalloproteinases (MMPs) has been implicated in the process of tumour metastasis. Cellular protein and RNA localisation techniques have been used to show that, whilst several MMP genes are expressed in both cancer and stromal cells, stromelysin 3 is expressed only in stromal fibroblasts adjacent to cancer cells. Immunohistochemical and in situ hybridisation evidence suggests that neoplastic cells can stimulate stromal cell MMP production either in a paracrine fashion or by a cell-cell contact mechanism. Using 2 different lengths of the human stromelysin 3 (ST3) gene 5' flanking sequence cloned upstream of luciferase and CAT reporter genes, we now show that human breast cancer cells can directly activate the ST3 promoter. The putative response element in the ST3 promoter, which lies between 0.46 and 3.4 kb upstream of the transcription start site, is able to effect a 2- to 3-fold increase in downstream gene expression. We further show that this transcriptional up-regulation definitely occurs via a paracrine, and possibly via a cell-cell contact, mechanism. Confirmation that this ST3 promoter activation results in ST3 gene induction of a similar magnitude was shown using Northern blotting of stimulated fibroblasts. Our data provide further evidence that cancer cells can induce fibroblast MMP expression and help to explain the in vivo expression pattern of ST3 in breast cancer.