The properties of rat liver dendritic cells (DC) were analyzed after collagenase digestion of the tissue and enrichment with density gradient centrifugation. The liver macrophages (Kupffer cells) were eliminated by adherence before the gradient centrifugation. The morphology of isolated DC in May-Grunwald-Giemsa (MGG) stained cytocentrifuge preparations resembled that of monocytes with certain dissimilarities. The expression of major histocompatibility complex (MHC) class II antigens (Ia) on DC was analyzed with the Staphylococcus aureus rosette method using a monoclonal antibody. The binding of anti-Ia antibody to rat liver DC was 3 times stronger than to passenger lymphocytes and 10 times stronger than to hepatocytes. All DC were Ia-positive tested with indirect immunofluorescence technique, and none of them were able to phagocytize antibody-coated human red cells. The DC did not express intracytoplasmic lysozyme, or surface Fc-receptors, and they all were negative in alpha-naphtylacetate esterase (ANAE) staining. Thus, although the dendritic cells of rat liver seem to belong to the monocytic series according to morphologic criteria, they were all negative when tested for typical monocyte/macrophage markers. The immunocenic potential of DC was analyzed by testing their ability to prime a naive recipient for graft rejection. The number DC needed for the priming was comparable with the number of spleen lymphocytes needed for an equivalent effect, indicating that the DC were highly immunogenic. The hepatocytes showed practically no immunogenic effect. Thus the immunogenic potential of the tested cells, i.e. their ability to induce accelerated transplant rejection, carries a good correlation with the expression of Ia-antigens on the cell surface.