SUMMARY The binding activity of fresh human thyroid homogenates for the long-acting thyroid stimulator (LATS) was shown to be principally associated with the particulate fraction. LATS binding activity (LAA) could be partially released into the soluble fraction by freezing and thawing of thyroid homogenates. By delaying freezing and thawing until the particulate fraction had been washed repeatedly it was possible to obtain highly purified preparations of LAA.