Recent studies have identified distinct but co-existing networks of resident tissue macrophages and MHC class II-positive DC present in tissues bordering the anterior chamber of the eye, a site classically regarded as 'immune-privileged'. As the DC network, present at approximately 500 cells/mm2, accounts for virtually all MHC class II immunostaining in these tissues and possesses potent capacity to stimulate primary allogenic responses in vitro, it is proposed that these cells may play an important role in immune surveillance of the anterior chamber. Tissue macrophage and DC population kinetics in the iris were examined by using X-irradiation exposure to interrupt the steady-state renewal of these cells by haematopoietically derived precursors. MHC class II-positive iris DC exhibited a half-life of approximately 3 days, a rapid turnover rate which closely resembled that of DC present in mucosal epithelia. In contrast, the resident tissue macrophage population displayed a considerably slower turnover (half-life of 10-12 days) comparable to that of epidermal Langerhans cells in the present study. Bone marrow transplantation studies confirmed the haematopoietic origin of the iris DC population. The present study provides the first estimates of the steady-state population kinetics of antigen-presenting cell populations in the iris and has important implications for understanding the role of these cells in immunological homeostasis of the anterior chamber.