The purpose of this study was to clarify the role of the matrix metalloproteinases (MMPs), collagenase (MMP-1), and gelatinase A (MMP-2), both of which are known to be involved in the development of gastric cancer, in peritoneal dissemination. The concentrations of MMP-1 and MMP-2 in the supernatant of mixed culture simulated peritoneal dissemination were measured in vitro with mesothelial cells and cancer cells using an enzyme-linked immunosorbent assay. The concentration of MMP-1 increased significantly after the contact culture was mixed with these two cells, in comparison with the non-contact mixed culture or the mesothelial cell culture alone. These results demonstrate that the production of MMP-1 derived from mesothelial cells was increased by contact with cancer cells. To clarify the effect of MMP-1 and MMP-2 on gastric cancer invasion, an invasion assay using matrigel was performed. After treatment with anti-MMP-1 monoclonal antibody (mAb) or anti-MMP-2 mAb, the number of matrigel-penetrating cancer cells was significantly reduced, indicating that MMP-1 and MMP-2 derived from mesothelial cells had a strong reaction to gastric cancer invasion. In conclusion, as MMP-1 showed a paracrine-like action responding to stimulus from cancer cells, it seemed to play an important role in the progression of peritoneal dissemination.