Recent immunomorphological studies have demonstrated the presence of distinct populations of resident tissue macrophages and major histocompatibility complex (MHC) class II+ dendritic cells within tissues lining the anterior chamber of rat, mouse and human eyes. The location of these cells in sites of potential contact with the aqueous humour-filled anterior chamber suggests that either of these cells may perform a role in immunosurveillance of this 'immune-privileged site'. The aim of the present study was to isolate highly purified dendritic cells and tissue macrophages from enzymatically disaggregated rat irides and to compare their relative capacity to stimulate unprimed T lymphocytes in vitro in a mixed leucocyte reaction assay. Dendritic cells freshly isolated from iris tissue exhibited a moderate ability to stimulate unprimed T lymphocytes. However, following 48 hr of culture in granulocyte-macrophage colony-stimulating factor (GM-CSF)-supplemented medium, MHC class II+ dendritic cells demonstrated a markedly enhanced stimulatory capacity that was identical to that of Langerhans' cells isolated from skin. Tissue macrophages isolated from rat iris, however, demonstrated little allostimulatory capacity, either when freshly isolated or following 48 hr of culture in GM-CSF. This study provides the first definitive evidence that MHC class II+ cells within tissues lining the anterior chamber are functionally equivalent to dendritic cells described in other tissues. These findings have important implications for our understanding of the mechanisms of immune surveillance within the anterior chamber of the eye.