An immunohistological study using monoclonal antibodies directed at specific membrane antigens of various inflammatory cells was carried out in order to evaluate the identity and topographic localization of the immuno-competent cells in an enucleated eye from a 6-year-old black patient with a three-month history of sympathetic ophthalmia. Correlative light and transmission electron microscopic examination of serial sections was also performed. The data demonstrated that the predominant cells within the choroidal infiltrate were T-lymphocytes (Leu 1+). T-cell subset analysis disclosed that most of these cells harbored specific antigenic determinants of the helper phenotype (Leu 3a+). A smaller proportion of the T cells demonstrated the specific determinants of the suppressor subtype (Leu 2a+). The helper/suppressor ratio varied slightly and ranged in most areas of the choroid between 3:1 and 4:1. Additionally, approximately 15% of the infiltrating lymphocytes harbored the Leu 14+ determinant specific for B cells. The latter were located in the outer choroid adjacent to the sclera. Very few natural killer (NK) cells (Leu 7+) were identified throughout the choroid. The granulomatous foci in the choroid were composed mainly of epithelioid cells and histiocytes expressing the OKM1+ and M221+ antigenic determinants on their membranes and demonstrating a high cytoplasmic nonspecific esterase activity (ANAE+). Within the Dalen-Fuchs nodules, similar to the choroidal nodules, there was a predominance of histiocytes and epithelioid cells (OKM1+, M221+, ANAE+), a few T-helper cells (Leu 1+, Leu3a+) and some OKM1-, M221- cells whose origin could not be determined. These findings were corroborated by electron microscopic observations of serial sections. Careful light and electron microscopic studies disclosed breaks in Bruch's membrane underlying some of Dalen-Fuchs nodules. In our opinion, these observations may be interpreted as the demonstration that Dalen-Fuchs' nodules and the choroidal granulomatous foci could be formed by identical cells of similar function and origin.