Materials and Methods.-Sources of plasma: The starting material usedin the purification procedure was lyophilized citrated plasma or lyophilized serum from two patients with hyperthy-roidism, exophthalmos, and consistently high titers of LATS-activity in the unaltered serum. One patient had pretibial myxedema; neither patient had an abnormal distribution of serum proteins as determined by paper electrophoresis. Purification procedure: The lyophilized plasma, or serum, was dissolved in distilled water to make a 2.5% solution. Ammonium sulfate was added to a 40% concentration at 40C. The pre-cipitate was allowed to form overnight in the cold, collected by centrifugation, then dissolved and dialyzed against 0.1 M sodium chloride for 12hr. The solution was then dialyzed 12 hr against 0.005 M phosphate buffer, pH 7.0. The euglobulin precipitate was removed, and the clear protein solution was passed through a 2.4-cm X 20-cm column packed withdiethylaminoethylcellulose (DEAE-cellulose) which previously hadbeen equilibrated with the phosphate buffer. The column load of the DEAE-cellulose (exchange capacity, 0.7 mEq/gm) was limited to 1.0 gm of protein. The unabsorbed protein fraction (gamma globulin) was dialyzed free of phosphate and was stored in the lyophilized state; the absorbed protein was removed by the passage of 0.2 M potassium phosphate through the column, dialyzed against 0.1 M sodium chloride, and lyophilized. Neutralization studies: The active gamma globulin was dissolved in saline, mixed with an equal volume of rabbit antibody to human 7S gamma globulin (Hyland Laboratories, Lot. no RP 763F), and incubated at 370C for 90 min. The solution was then held at 40C overnight. Although no visible precipitate formed, the solution was centrifuged and the upper portion removed forassay. A control sample of active gamma globulin was treated in an identical fashion with the exception that saline was substituted for the rabbit antibody. Reduction of the active gamma globulin: The method for the reduction and fractionation of gamma globulin described by Fleischman et al. 8 was used with minor modifications. A 2% solution of