Murine Kupffer cells, the tissue macrohages of the liver, were isolated by collagenase digestion, differential sedimentation over Metrizamide, and glass adherence. The resultant cell population was more than 86% phagocytic, and 95% of cell stained positively for α-naphtyl butyratc esterase activity. The cells also had cell surface receptors for complement (C) and the Fc portion of IgG. In additon, a large proportion of Kupffer cells was shown to bear Ia antigens: about half of the cells bore I-A subregion-encoded antigens. Kupffer cell populations were capable of reconstituting antigen-stumulated proliferative responses of antigen-primed, macrophage-depleted, lymph node T cells. The ability to reconstitute proliferation was enriched in the adheren population and was resistant to radiation and treatment with an anti-Thy antiserum and C. We conclude that isolated murine Kupffer cells bear the Ia phenotype of accessory cells that function in antigen presentation and that Kupffer cells can paricipate in the induction of antigen-specific immune responses. These data suggest that Kupffer cells may play a role in modulating responses to enterically derived antigens.