The metabolism of intravenous human IgG preparation (C-425) was studied by the use of 125I-C-425 in rats and rabbits after intravenous injection of the compound particularly with regard to changes in the levels of radioactive compound or metabolites in the blood as a function of time as well as levels excreted in urine and feces. Brain uptake index of 125I-C-425 was estimated by a slightly modified Oldendorf's method. 125I-Polyglobin was used as the control compound. The half-life of blood clearance of 125I-C-425 radioactivity after the 2nd day was about 8 days in rats and about 5 days in rabbits, respectively. Within a relatively short period of 1 hour, a rapid decrease of blood concentration of radioactivity was recorded. This was due to the transfer of labelled compound from blood into the extravascular compartment. A high concentration of radioactivity in the blood was maintained for one week after intravenous injection of the labelled compound in both rats and in rabbits. Most of the radioactive compound or metabolites were excreted in the urine and relatively little in feces. Excretion in urine of rats reached about 22% for 125I-C-425 and about 17% for 125I-Polyglobin. The excretion rate slowed down over 24 hours. The total recovery rates in rats were about 40% for 125I-C-425 and 125I-Polyglobin and about 55% in rabbits for both drugs. The permeability through the blood-brain barrier was found to be negligible for both drugs.

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