In the present study, we investigated the effect of the RANTES-mediated interaction between gastric carcinoma cell lines and peripheral blood mononuclear cells (PBMCs) in tumor progression. RANTES production in PBMCs stimulated by highly metastatic cancer cell line-conditioned supernatants was higher than in those stimulated by a less metastatic gastric cancer cell line-conditioned supernatant. RANTES receptors were expressed in PBMCs, but not in those cancer cell lines; therefore it was suggested that RANTES might affect PBMCs but not cancer cells. Matrix metalloproteinase (MMP)-9 expression in PBMCs was examined. Similar to RANTES production, MMP-9 expression in PBMCs stimulated by highly metastatic cell line-conditioned supernatants was higher than in that stimulated by a less metastatic cell line-conditioned supernatant. Invasion assays of gastric cancer cell lines were performed. Cancer cells cultured with PBMCs invaded into Matrigel more frequently than those without PBMCs. This invasive activity was highly inhibited by an anti-RANTES antibody. These results suggest that tumor cells can acquire the potential for invasion by cooperating with PBMCs and RANTES plays an important role in the interplay between tumor cells and PBMCs. It is thus thought that RANTES might be a candidate molecular target in the therapeutic strategy for gastric cancers.