The expression of transferrin receptors marked by the monoclonal antibody OX-26 and localisation of iron was studied in epiplexus cells in the lateral ventricles of different aged rats subjected to the challenge of the bacterial toxin, lipopolysaccharide, and treatment with interferon-gamma. Transferrin receptor expression in epiplexus cells was extremely low in normal rats, but was vigorously elevated in rats receiving intraperitoneal injections of lipopolysaccharide (LPS); other antigens such as complement type 3 receptors immunostained with OX-42 and macrophage antigen marked by ED-1 were also notably augmented. After 6 successive intraperitoneal injections of interferon-gamma, the expression of transferrin receptors showed only a moderate increase. Using Perls' staining method, some iron-containing epiplexus cells were observed in early normal postnatal rats, but their number was markedly decreased with age. After LPS administration, a moderate increase in the number of iron-containing epiplexus cells was observed in different aged rats. The significance of the upregulation and vigorous expression of transferrin receptors on epiplexus cells is speculative. One possible explanation would be that they facilitate an increase in the acquisition of iron by these cells for storage, oxidative killing and/or immunological activation. The localisation of iron in the choroid plexus in the lateral ventricles suggests that the latter may function as an important storage organ for iron. Present results also suggest the presence of an efficient transport system for the transfer of iron across the interface of the blood and cerebrospinal fluid.