Summary.-Interactions between TA3 mammary-carcinoma cells and liver cells were studied with the electron microscope in mouse livers that had been perfused with a defined medium containing the tumour cells.

Liver of (C57BL X DBA)F1 mice were perfused in situ with a synthetic hemoglobin-free medium, to which murine lymphosarcoma cells were added. All cells were arrested. They were only found in the sinusoids, predominantly in periportal areas. Many lymphosarcoma cells penetrated the walls of the sinusoids with protrusions that extended into and through the endothelial cells. The protrusions often also invaded hepatocytes, and some cells migrated out of the sinusoids. The percentage of the cells that penetrated endothelium was constant and reproducible (68 +/- 4%) in experiments lasting longer than 90 minutes, but the percentage of these cells that also invaded hepatocytes varied greatly. Parellel experiments in vivo yielded similar results, except that the number of cells that invaded hepatocytes was generally much lower. The advantages of the perfused liver as a model for experimental study of the invasion mechanism were evaluated.

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Previously we have described the infiltration of lymphosarcoma cells into hepatocyte cultures. The interaction between tumor cells and hepatocytes was comparable to that occurring during the formation of liver metastases. Presently we report that antigen-activated T lymphocytes, but not unstimulated T cells, infiltrate hepatocyte cultures in a manner comparable to the lymphosarcoma cells. Thus, liver-colonizing lymphoid tumor cells and activated non-transformed T cells apparently have common characteristics, that enable them to infiltrate between liver cells. A comparative study of activated and non-activated T cells may aid in elucidating these characteristics. In the intact liver activated lymphocytes did not infiltrate, probably because they were not arrested.