A culture model for invasion of rat mesothelial cell layer by rat ascites hepatoma cells has been developed. By using this quantitative model, the preculture with macrophages (0.1 less than macrophage/tumor cell less than 1.0) was found to enhance both the in vitro and in vivo invasive potentials of the tumor cells. This potentiation appears to be mediated partly by oxygen radicals generated by the cocultured macrophages. The in vitro invasive capacity was also augmented by pretreating the tumor cells with TGF-beta or with activated platelets. A factor with anti-invasive potential (IIF) was extracted from rat liver. It inhibited the directed migration but not the growth of the tumor cells and was effective on their in vivo invasion and metastasis, as well.