We have recently established a human renal cell carcinoma KG-2 line that is tumorigenic in the subcutis (ectopic) and kidney (orthotopic) of nude mice but spontaneously metastasizes to the lung only after orthotopic implantation. KG-2 cells growing in the kidney (orthotopic) and lung metastases secreted higher levels of gelatinase than did cells growing in the subcutis (ectopic). We examined whether organ-specific fibroblasts play a role in the regulation of gelatinase production and invasion by renal carcinoma cells. The gelatinase level in the culture supernatants of KG-2 cells was increased by their cultivation with mouse kidney or lung fibroblasts. In contrast, cocultivation of KG-2 cells with mouse skin fibroblasts resulted in a significant reduction of gelatinase activity. Similar results were obtained by culturing KG-2 cells in the media conditioned by the different mouse fibroblasts. We, therefore, investigated effects on KG-2 cells of cytokines and growth factors known to be produced by fibroblasts of various origins. Of ten cytokines and growth factors tested, basic fibroblast growth factor, hepatocyte growth factor, and transforming growth factor-beta 1 (TGF-beta 1) stimulated gelatinase expression by the cultured KG-2 cells. Parallel immunohistochemical analyses revealed that mouse kidney and lung fibroblasts produced higher levels of TGF-beta 1 than did skin fibroblasts. These results indicate that gelatinase production by KG-2 renal cell carcinoma cells is influenced by the organ microenvironment. Specifically, organ-specific fibroblasts regulate the production of degradative enzymes by KG-2 cells and, hence, profoundly influence their invasive and metastatic capacity.