The immunoreactivity for epidermal-type fatty acid binding protein of epidermis type (E-FABP) was selectively localized in the epithelial cells of both cortex and medulla of mouse thymus. The cortical epithelial cytoreticulum was clearly visible with the intense immunoreactivity and the immunoreactive cytoreticulum extended intricately throughout the thymic cortex to enclose thymocytes. In the thymic medulla, the immunoreactivity was variable in intensity among the epithelial cells and there was a tendency that epithelial cells containing more numerous tonofilament bundles were less immunoreactive. Considering the possibility that FABPs function as intracellular carriers for unsaturated long chain fatty acids, the present finding suggests that E-FABP in the thymic epithelial cells, especially the cortical ones because of their extensive location, are intimately involved in the metabolic processes of fatty acids including production of bioactive substances, such as prostaglandin and leukotriene, which are known to exert some regulation of thymic immune responses.