Lymphoid dendritic cells and pituitary folliculo-stellate cells have many morphological and immunocytochemical characteristics in common. They both have a stellate morphology, they are specifically detected by the presence of MHC class II determinants as well as the Ca (2+)-binding protein S100, they also produce the cytokine interleukin 6. A panel of monoclonal antibodies directed against murine and rat macrophages and/or dendritic cells was used to study the presence of dendritic and folliculo-stellate cells in the mouse and rat anterior pituitary. In the mouse pituitary stellate cells are detected with a monoclonal antibody against the dendritic cell aminopeptidase, but these cells display no S100 protein immunoreactivity. In the rat pituitary there were many S100 protein positive folliculo-stellate cells as well as a few macrophage-like cells, whereas stellate cells that express MHC class II markers were found in both mouse and rat anterior pituitaries. The present data suggest of a homology between lymphoid dendritic cells and pituitary folliculo-stellate cells, or at least a subgroup of the latter cells.