This work is to study the in vivo biological distribution of ¹³¹I-labeled mouse/human chimeric monoclonal antibody (¹³¹I-chTNT) in patients with pulmonary metastases from differentiated thyroid carcinoma. Ten patients with differentiated thyroid carcinoma were injected intravenously with a single dose of ¹³¹I-chTNT (5 MBq·kg^-1 body weight). Radioactivity of blood and urine samples was measured at different time points. The in vivo stability and the metabolic status of ¹³¹I-chTNT were detected with supersaturated trichloroacetic acid. Continuous imaging was performed to outline the region of interest (ROI) and estimate the intake level on the whole body, major organs and tumor lesions at different time points. The serum time-radioactivity curve of ¹³¹I-chTNT accorded with the two-compartment model after a single intravenous injection: T_1/2(h)=65.28±14.83, AUC_0-t(MBq·h·mL^-1)=8.93±1.32, AUC_0-∞(MBq·h·mL^-1)=10.58±2.19, and CL(mL·min^-1·kg^-1)=1635±359. The time-radioactivity percentage curve of ¹³¹I-chTNT urine excretion accorded with the one-compartment model after a single intravenous injection: T_1/2(h)=99±10, and accumulative (31±9) % radioactivity of the injected dose was excreted in urine in one week. The percentages of serum ¹³¹I-chTNT in radioactive components at 24, 48 and 72 h were over 95% and it was still (88±7) % at 168 h. As for chemical composition of radioactive substances in urine, radioactivity in urine samples originated from free ¹³¹I by 100%. Radioactivity of ¹³¹I-chTNT after intravenous administration was mainly concentrated in the lung and liver, least in the brain. Radioactivity of tumor tissues reached the maximum at 24 h and the tumor/normal tissue (T/N) ratio reached the maximum (1.28~3.83) during 3~7 d. The characteristics of in vivo biological distribution of ¹³¹I-chTNT in patients with pulmonary metastases from differentiated thyroid carcinoma are favorable for its therapeutic application for the metastasis tumors.