Although the function and significance of tumor-infiltrating dendritic cells (TIDC) in the immune response to tumor have never been clearly demonstrated, their location suggests that they play a critical role in the presentation of tumor antigen to specific T cells. We studied the morphological and functional characteristics of interstitial dendritic cells (DC) located inside tumors obtained by injection of cancer cells into syngeneic rats. Single and double immunostaining of tumor sections revealed a dense network of cells which expressed class II major histocompatibility complex (MHC II) molecules. Cell morphology and surface markers were characteristic of DC populations in other tissues. These DC were in close contact with tumor cells and increased in number as the tumor grew larger. Unexpectedly, a subpopulation of morphologically characteristic TIDC expressed both CD8 and MHC II molecules. TIDC were purified from tumors by gradient centrifugation and immunobeads and characterized by morphology, ultrastructural study and surface markers studied by flow cytometry. TIDC were negative for the CD5 molecule (a pan T cell marker), and were not labeled with 3.2.3 monoclonal antibody (mAb) (an NK cell marker) or with Ki-M2R mAb (a macrophage marker). A subpopulation of TIDC expressed the CD8 molecule, confirming the in situ results. TIDC expressed high levels of class I and class II MHC molecules and the adhesion molecule ICAM-1. This expression is compatible with effective antigen presenting function. Purified TIDC triggered rapid and high levels of proliferation of tumor-immune T cells in vitro, demonstrating the potential of these cells to constitutively process and present tumor-associated antigens.