1. 1. The distribution of I131 was studied in nonimmune guinea pigs following intravenous administration of trace labeled I131 diphtheria toxoid.2. 2. The loss of I131 from the plasma could be attributed to three exponential components. The major fraction (85–90%) was removed at a rate of 69% per hr (T-? = 1.0 hr) and the remaining 10–15% at two rates, one with a half-time of 5.5 hr and the other with a 37-hr half-time.3. 3. The excretory loss of I131 from the guinea pigs occurred with two exponential rates, about 70% was excreted with a half-time of 9 hr and the remaining 30% with a half-time of about 100 hr.4. 4. There was marked localization of I131 in the liver, spleen and kidney between 2 and 14 hr after administration of the toxoid, as determined by tissue to plasma ratios and I131 recovery calculations.5. 5. Both toxin and toxoid are indistinguishable from each other when the concentration time course of I131 in the plasma and excreta are studied. The tissue distribution, however, differs significantly. I131 diphtheria toxin appears to be widely and diffusely distributed throughout all the tissues whereas I131 diphtheria toxoid appears to localize in the reticuloendothelial system (liver and spleen).
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