Macrophages are a major population of thymic cells along with lymphocytes and epithelial cells. They are distributed in an apparently random manner throughout the cortex and medulla. Thymic macrophages express all of the various identifying characteristics associated with macrophages throughout the body including expression of a high level of class-I and II MHC products. They account for at least 99% of thymic Ia positivity. Thymic macrophages exhibit the property of binding thymic lymphocytes, and in some cases those bound lymphocytes are phagocytosed. This can result in the production of 'nurse cells'. Thymic macrophages can induce maturation of thymic lymphocytes and studies with non-thymic macrophages suggest that the macrophage-induced maturation is MHC restricted. The various relationships between lymphocytes and macrophages in the thymus suggest that the interaction between those two cell types is crucial to thymocyte maturation, generation of MHC restriction and generation of tolerance to some self-antigens.