The thyrotropin (TSH) receptor is implicated in the pathogenesis of thyroid-associated ophthalmopathy, but its presence has not been demonstrated in orbital tissues. Binding of 125I-TSH to porcine orbital connective tissue and thyroid membranes was studied. At pH 7.4 125I-TSH bound to thyroid membranes with high affinity, but not to orbital connective tissue membranes. At pH 5.2, binding of 125I-TSH was evident on thyroid and orbital connective tissue membranes, although of low affinity. Low affinity binding at pH 5.2, could also be demonstrated on orbital fibroblasts; binding to membranes prepared from extraocular muscle, or from abdominal adipose tissue was not detected. Immunoglobulin G from healthy subjects (n = 17) and patients with ophthalmopathy (n = 19) were tested for inhibition of 125I-TSH binding to membranes. As anticipated patients' IgG exhibited greater inhibition than normal controls with thyroid membranes at pH 7.4 (p < 0.001). Similar results were obtained with thyroid membranes (p < 0.05), and orbital connective tissue membranes at pH 5.2 (p < 0.001). Significant correlations were found when percentage inhibition by IgG was compared between: thyroid membranes at pH 7.4 and thyroid membranes at pH 5.2, orbital connective tissue membranes at pH 5.2 and thyroid membranes at pH 7.4, and orbital connective tissue membranes at pH 5.2 and thyroid membranes at pH 5.2. A low affinity TSH-binding site is present in orbital connective tissue, and is recognized by IgG's from patients with ophthalmopathy which suggests that TSH receptor antibodies may target the orbit.