HLA-DR positive cells infiltrating Graves' thyroid tissue were examined for their expression of cell-specific immunological markers using light and electron microscopic immunostaining of frozen sections and isolated open thyroid follicles. Graves' glands (n = 21) were enriched of CD68 and Leu-M5/CD1 lc positive monocytes/macrophages as well as CD4 and CD8 lymphocytes. These cell types were distributed juxtafollicular as well as in other tissue areas. Only the RFD-1 antibody, considered to label antigen-presenting cells including dendritic cells, identified cells invariably located close to the interstitial follicular surface. After follicle isolation, RFD-1 cells were enriched compared to Leu-M5 cells and exclusively adherent to the follicular epithelium. The plasma membrane of RFD-1 positive cells were in intimate contact with the basolateral membrane of the thyrocytes, sometimes extending deeply into the intercellular space of the epithelium. Parallel labelling experiments suggested that the follicle-adhering RFD-1 cells also expressed HLA-DR. Our findings show that in human thyroid glands with Graves' disease RFD-1 positive cells with a dendritic morphology establish direct contact with the follicular epithelium. In view of the fact that both HLA-DR and RFD-1 are associated with antigen-presenting functions it is suggested that a direct interaction of dendritic cells with thyrocytes might be an important component of the autoimmune reaction in Graves' disease.