Macrophages play a role in the host defence against cancer. Little is known about changes in macrophage populations during early metastatic growth. To evaluate the distribution, number and phenotype of macrophages in the development of hepatic metastases in a rat model (Wag/Rij rats and syngeneic CC531 colon carcinoma cell line), an immunohistochemical study was performed with the monoclonal antibodies ED1 (monocytes, and all macrophages), ED2 (resident tissue macrophages, like Kupffer cells) and ED3 (a subpopulation of macrophages which may play a role in the recruitment of lymphocytes). OX19 and His14 were used to identify lymphocytes. In this study a new monoclonal antibody CC52 is described, which recognizes the CC531 tumour cell line. Liver metastases were induced by injection of CC531 colon carcinoma cells into a mesenteric vein. Rats were killed at various intervals. Results show three major macrophage populations during hepatic tumour growth: (1) on day 3, infiltrates are observed around the micrometastases, which contain mainly newly recruited macrophages (ED1+ and ED2-); (2) after 7 days, ED3-positive (ED3+) macrophages together with T lymphocytes are found in the infiltrates; (3) an increase in the number of ED2-positive (ED2+) Kupffer cells is observed in the liver parenchyma after 14 days. In conclusion, the present results suggest that various populations of macrophages, newly recruited (ED1+) as well as resident Kupffer cells (ED2+), are involved in the immune response against tumour cell deposits in the liver.