Liver-derived high density lymphocytes (Matsunaga cells) have been detected as members of resident T cells in the mouse liver. In this study, we assessed the immunological functions of liver-derived high density lymphocytes of BALB/c mice in comparison with those derived from spleen and peripheral blood. Liver-derived high density lymphocytes proliferated in response to the syngeneic and allogeneic mixed lymphocyte reaction, as well as those derived from spleen and peripheral blood. The allo-activated cytotoxic T lymphocyte activity of liver-derived high density lymphocytes against PHA-blasts of C57BL/6 mice was lower than that of spleen- and peripheral blood-derived lymphocytes. The suppressor activity of syngeneic- or allo-activated high density lymphocytes of the liver, spleen, and the peripheral blood was assessed by measuring their suppressive effect on the proliferation or on the generation of allo-specific cytotoxic activity in allogeneic mixed lymphocyte reaction. The suppression was concentration-dependent and strongest in liver-derived lymphocytes.