The monocyte procoagulant activity (PCA) production assay has been shown to be a good parameter of cell‐mediated immunity. We have studied antigen‐specific PCA production in peripheral blood mononuclear cells from patients with Graves’disease to determine the effect of the treatment on the cell‐mediated immune response. Peripheral blood mononuclear cells from patients with untreated or relapsed Graves’disease produced significantly greater PCA with thyroid antigen stimulation than those from normal subjects. Patients both on antithyroid drugs in the hyperthyroid state and within 3 months post‐131I therapy also produced significantly larger amount of PCA than normal subjects. However, there was no significant difference in PCA production with thyroid antigen stimulation between normal subjects and patients on anti‐thyroid drugs in the euthyroid state, or patients over 3 months post‐131I therapy. The ratio of positive to negative PCA production in patients on anti‐thyroid drugs in the euthyroid state or over 3 months post‐131I therapy was significantly lower than in untreated or relapsed Graves’disease patients. Mononuclear cells from patients on propylthiouracil responded to propylthiouracil
in vitroby production of PCA. Cells from normal subjects, untreated Graves’disease patients, or patients with Hashimoto's thyroiditis did not produce PCA with propylthiouracil stimulation. Mononuclear cells from patients who were on propylthiouracil for more than 3 months produced greater PCA than those on the drug for less than 3 months, suggesting sensitization of lymphocytes to propylthiouracil during the course of treatment. However, after 131I therapy, they gradually became unresponsive to propylthiouracil. This study has shown that the activity of the antigen‐specific response assessed by PCA production in mononuclear cells from Graves’disease patients declined after treatment, suggesting that the treatment exerted immunomodulatory effects.