Artificial liver metastases were produced by injecting in vitro cultivated colonic cancer cells into the mesenteric vein of syngeneic rats. An immunohistologic study of the inflammatory host cells infiltrating the tumors was carried out from 1 to 46 days after tumor cell injection, using a set of monoclonal antibodies labelling tumor cells, macrophages or lymphoid cells subpopulations. In small metastases, 3-7 days after tumor cell injection, tumor cell foci were surrounded by a dense corona of inflammatory cells, predominantly T lymphocytes and NK cells. This inflammatory corona regressed when metastases grew larger and completely disappeared around the large metastases found more than 28 days after cell injection. Inside the metastases, the tumor cells were mixed with a small number of cells with the characteristic labelling of macrophages (KiM2R+, W3/25+); however, the inside macrophages are OX8+ in contrast to macrophages found in normal liver and in the peritumoral corona. The progressive disappearance of the coronal inflammatory cells suggests that cancer cells are able to produce or to induce other cells to produce anti-inflammatory agents inhibiting the accumulation of macrophages and lymphoid cells around the tumors.