No measurable amounts of matrix metalloproteinases (MMPs) were produced by human breast adenocarcinoma cell lines MCF-7 and BT-20 in culture. When MCF-7 cells were co-cultured with human dermal fibroblasts enhanced production of precursors of MMP-1 (interstitial collagenase), MMP-2 (gelatinase A), MMP-3 (stromelysin 1) and tissue inhibitor of metalloproteinase type 1 (TIMP-1) was observed. Immunohistochemical studies indicated that these pro-MMPs originated primarily from the fibroblasts, suggesting that MCF-7 cells have a stimulatory effect on stromal cells to produce at least three pro-MMPs and TIMP-1. BT-20 cells also enhanced the production of pro-MMP-2 and TIMP-1 in the dermal fibroblasts, but not of pro-MMP-1 and pro-MMP-3. Normal mammary epithelial cells promoted only TIMP-1 production. To investigate further the stimulatory factors from MCF-7 cells, the conditioned medium and the cell membrane were prepared and examined. The cell membrane fraction enhanced the production of pro-MMP-1 and -3 and TIMP-1, but not of pro-MMP-2. The conditioned medium, on the other hand, augmented the production of all four proteins in the fibroblasts. These observations suggest that breast adenocarcinoma MCF-7 cells in culture produce both soluble and membrane-bound factor(s) which stimulate the production of pro-MMPs and TIMP-1 in neighbouring stromal cells, but the factor(s) released into the medium and that associated with cell membranes are probably different. Such communication between the normal and malignant cell types may, in part, assist the cancer cells to invade and metastasise.