A multiplicity of techniques has arisen for the measurement of the immunoglobulins specifically associated with Graves' hyperthyroidism, which has led to difficulty in assessing the clinical significance of these immunoglobulins. In this study of untreated toxic Graves' disease we have taken two assays (LATS protector and thyrotrophin binding inhibiting immunoglobulin, TBII) which depend upon the measurement of binding to thyroid cell membranes, and correlated them with a bioassay which measures as the index of activity the biologically relevant end point of T3 secretion from thyroid tissue. The frequency of positive responses obtained with each method was similar to that of previous studies; combining the results a positive response was found in 100%. No significant correlation was found between any of these three assays. T3 to assess which method relates most closely to circulating thyroid hormone levels in vivo. A highly significant correlation was obtained for TSAb measured by T3 secretion with both total serum T3 and total serum T4 levels. A positive correlation was also found for LATS protector, but not for TBII. We conclude that although these assays are specific for Graves' immunoglobulins, they are measuring closely related but not identical immunoglobulin activities.