Determination of antibody through the use of I131 label; experiments with equine diphtheria antitoxin.

Equine diphtheria antitoxin has been labelled with iodine containing I131 in the ratio of 0.25 to 13.0 atoms of iodine per globulin molecule. The smaller amounts of iodine produce little or no change in precipitable antitoxin. The rate of iodination of equine antitoxic globulin is 90 to 94 per cent of that of the average serum protein. The feasibility of direct measurement of antibody protein by means of an I131 label is supported.