We have studied the significance of antibody dependent cell-mediated cytotoxicity (ADCC) against human orbital fibroblasts (OF) and eye muscle (EM) cells in the pathogenesis of the orbital autoimmune reactions of Graves' ophthalmopathy (GO). Possible roles of Class II MHC antigen expression on the surface of orbital target cells and of gamma-interferon (gamma-IFN) modulation of ADCC were also studied. Both OF and EM expressed HLA-DR antigen when stimulated with gamma-IFN and phytohemagglutinin, but not spontaneously, and not by thyroid stimulating hormone or alpha-IFN. Intrathyroidal T cells from a patient with GO induced greater DR expression on both OF and EM cells than equal numbers of her peripheral blood T cells. gamma-IFN treated EM and OF were more susceptible to lysis in ADCC assays than untreated targets. gamma-IFN also enhanced lysis in ADCC assays by an effect on the killer cell population. On the other hand treatment with alpha-IFN, which is a potent inducer of Class I antigen expression, did not affect the susceptibility of target cells to lysis in ADCC. When sera from patients with GO were tested in ADCC, tests were positive (% specific lysis greater than mean + 2 s.d. for normals) in 10 of 20 patients with EM cells, but in only two of 25 with OF. The degree of killing of EM cells was significantly positively correlated to that of abdominal skeletal muscle cells and, to a lesser degree, normal thyroid cells, but not OF. In sera showing killing of EM cells and OF, ADCC activity against EM cells was absorbed by preincubation with EM and orbital connective tissue membranes but not thyroglobulin and, conversely, lysis of THY cells was absorbed by preincubation of positive sera on monolayers of THY and EM cells and OF, but not vascular endothelial (VE) cells. Finally, killing of 51Cr labelled EM cells was inhibited by addition of unlabelled ('cold') thyroid cells, EM cells and OF, but not VE cells. Our findings suggest that ADCC is likely to be an important mechanism for the eye muscle cell damage of GO, but not for the associated orbital connective tissue inflammation. Since ADCC is not MHC-restricted the enhanced lysis of HLA-DR positive target cells presumably reflects other effects of gamma-IFN treatment on both the killer cell population and the target cells.