Tie-1 is a transmembrane receptor expressed in vascular endothelium during angiogenic events and during embryonal growth in most mammalian species. The monoclonal antibodies (mAbs) used here are generated against the extracellular part of the Tie-1 receptor protein. We analyzed the specific binding of ¹²⁵I-labeled Tie-1 mAbs in the metastatic tumor model in mouse and in human serum samples to determine the possible use of the Tie-1 mAbs in detecting malignant growth. The in vivo biodistribution of ¹²⁵I-Tie-1 mAbs (IgG1) was evaluated in the mouse model. The same Tie-1 mAbs were used to analyze Tie-1 in patient serum samples. A high accumulation of two ¹²⁵I-Tie-1 mAbs, clones 10f11g6 and 3c4c7, was detected in the lung metastases of mouse tumor model. The uptake in the metastases is 12% injected dose/gram (ID/g; 10f11g6) and 7.8% ID/g (3c4c7) at 96 h after the injections of ¹²⁵I-Tie-1 mAbs, and the accumulation to the blood is 7% and 5% ID/g with the two mAbs, respectively. The finding directed us to analyze serum samples from lung, ovarian, and breast cancer patients. High levels of Tie-1 were detected in samples related to new pelvic or thoracic metastases in patients. Here we connect the Tie-1 levels in serum to tumor progression. The results suggest that Tie-1 mAbs can be used as a surrogate marker of progressive disease.