The current knowledge of thyroid-stimulating antibody (TSAb) and its significance in Graves' disease is reviewed under 4 headings. 1) Methods of assay; these are categorized as thyroid-stimulation or thyrotropin-receptor-modulation type methods. The latter are convenient but non-specific and the former are inconvenient but specific. The use of guinea pig fat cell membranes as a source of receptor for thyrotropin may improve the specificity of the thyrotropin-binding inhibition (TBI) system. 2) Immunochemistry of TSAb; evidence for the restricted heterogeneity, or oligoclonality, of the antibody as it occurs in some sera, viz. selected for the very high titer, includes a relatively constant pI on isoelectric focussing, restriction to IgG1 and having only lambda or k as the light chain. 3) Are antibodies other than TSAb pathogenic in hyperthyroidism? data are provided indicating the presence in one serum of an antibody that inhibits the action of TSAb in vitro. Clinically this novel antibody caused delayed onset of neonatal hyperthyroidism in 2 children. The prevalence of the antibody and its general clinical significance are unknown, but ways of testing for its presence are reviewed. 4) Clinical significance of the assay of TSAb; TSAb occurs in at least 90% of patients but should not be necessary for the diagnosis of Graves' disease. Its persistence at the end of a course of antithyroid drugs predicates relapse; a high level on first diagnosis may forecast such persistence and be an indication for ablative therapy for hyperthyroidism. A high level of TSAb in the third trimester of pregnancy is a reliable index of neonatal hyperthyroidism. It should be recognized that there is a marked tendency for TSAb values to fall throughout the course of pregnancy.