Thyroid-stimulating antibody (TSAb) and TSH binding inhibitor immunoglobulin (TBII) were measured serially in 10 patients with Graves' disease at the time of postpartum onset (n = 2) or relapse (n = 8) of Graves' thyrotoxicosis and in 5 patients with Graves' disease who were in remission and had no postpartum relapse of Graves' thyrotoxicosis. TSAb was measured by a sensitive cAMP accumulation assay using FRTL-5 cells, and TBII was determined by radioreceptor assay. In no patient with either recurrent or new onset postpartum hyperthyroidism did the serum free T3 index (FT3I) rise before the free T4 index (FT4I). Of the 10 patients who had postpartum thyrotoxicosis, concomitant increases in serum FT4I and FT3I, and TSAb and TBII were observed in only 1 patient. Increases in TSAb and TBII after those in FT4I and FT3I occurred in 6 patients. In 1 patient, an increase in TBII was associated with the occurrence of thyrotoxicosis, but TSAb increased 1 month later. In the other 2 patients, a TSAb increase was followed by the development of thyrotoxicosis, but TBII increased later. In 3 of these 10 patients, the increased serum FT4I and FT3I values decreased spontaneously, whereas the TSAb and TBII levels increased continuously. No positive test or increase in TSAb or TBII was found in the 5 patients with Graves' disease who did not have a postpartum relapse of thyrotoxicosis. These data indicate that postpartum initiation of Graves' thyrotoxicosis is not always associated with an increase in circulating anti-TSH receptor antibodies and that such parameters are poor indicators of thyroid function. Intrathyroidal humoral or cell-mediated immunological mechanisms may also be involved in mediating thyrotoxicosis in Graves' disease.