Long-acting thyroid stimulator (LATS) was found in the plasma in each of 7 subjects with circumscribed, pretibial myxedema, which in every instance had developed after surgical or radioiodine treatment of thyrotoxicosis. Using a potent plasma from one of these subjects, a bio-assay method for the quantitative measurement of LATS was developed which was used in subsequent fractionation and recovery experiments. By fractional precipitation of the proteins of LATS plasma by acid potassium phosphate, followed by gel filtration of the active fraction through diethylaminoethyl-Sephadex, LATS was eluted with a single protein moiety and was recovered in 60–80% yield with an 8-fold increase in purity. Analysis by the methods of ultracentrifugation, paper electrophoresis, immunoelectrophoresis, double diffusion in agar, and sucrose gradient separation confirmed the presence of a single protein which was identified as a 7S gamma globulin. Incubation of LATS with antihuman-7S antibody or digestion of LATS by papain destroyed its biologic activity. Incubation of LATS plasma with dog or human thyroid tissue reduced the apparent yield of LATS. Pretreatment of the test animal with actinomycin D or puromycin markedly inhibited the assay response to administered LATS and TSH. Oral corticosteroid therapy resulted in a marked fall in plasma LATS concentration, and improvement of pretibial lesions, but relapse occurred upon drug withdrawal. Marked improvement of the pretibial lesions was achieved by local application of a corticosteroid cream under an occlusive dressing. These observations are considered in relation to an antibody role for LATS.