Abstract: In order to study the impact of Ricin on the pharmacokinetics of ¹²⁵I-MIL50 and to investigate the tissue distribution and excretion of ¹²⁵I-MIL50 in Wistar rats, MIL50 was labeled with ¹²⁵I using the Iodogen method. Then, the concentration of ¹²⁵I-MIL50 in serum、tissues、body fluids and excretions was measured by TCA method at different time. The results showed that ¹²⁵I-MIL50 was eliminated faster after 14 days in the Ricin administrated group. The concentration of ¹²⁵I-MIL50 in serum was always higher than that in other tissues. The level in kidney and bladder were high and in brain, spine and fat were low. The cumulative excretion rate of ¹²⁵I-MIL50 was 62.6% in urine, and 15.5% in feces within 27 days. Ricin could fasten the elimination of ¹²⁵I-MIL50 when the concentration of ¹²⁵I-MIL50 was low in Wistar rats. It might because of the interaction between antigen and antibody. ¹²⁵I-MIL50 had no specific combination with tissues and it could hardly entered into lipophilic tissues. Urinary excretion represented the major pathway of elimination of ¹²⁵I-MIL50. The results of the study provide a reference for clinical trials and drug administration program.