To evaluate the expression of the Tie2/Tek tyrosine kinase receptor in tumor blood vessels, we examined Tie2lacZ⁺/RAG1⁻ mice. There was considerable heterogeneity (Tie2-negative, Tie2-positive, or Tie2-composite blood vessels) in subcutaneous xenografts of human colorectal carcinoma (HCT116; 97.5% Tie2-positive vessels) versus human melanoma (WM115; 75.9% Tie2-positive vessels). Similar patterns of Tie2 expression occurred in abdominal metastases derived from the same cell lines. Immunostaining for endothelial markers and Tie2 revealed that endogenous protein levels corresponded with transgene activity. Endothelial cells were confirmed to be of mouse origin through triple immunofluorescence staining with mouse antiserum to human nuclei, isolectin GS-IB₄, and anti-Tie2. Similar Tie2 heterogeneity was observed in clinical specimens from a variety of human cancers, including malignant melanoma and colorectal carcinoma. We also examined the effect of Tek-Delta Fc anti-angiogenic therapy on tumor growth and Tie2 expression patterns in HCT116 and WM115 subcutaneous xenografts. Tek-Delta induced extensive tumor regression in HCT116 tumors and concomitant reductions in Tie2-expressing blood vessels. However, no significant responses were seen in Tek-Delta-treated WM115 tumors. Thus, vascular heterogeneity of Tie2 expression is cancer-type specific, suggesting that the tumor microenvironment and/or direct cancer cell interactions influence Tie2 endothelial expression.