To develop effective therapeutic strategies aimed at treating tumor metastasis, critical steps in this process must be better understood. For this purpose we have established a new model to visualize and quantify early metastasis. Murine CT-26 colon adenocarcinoma cells were stably transfected with green fluorescent protein (GFP). Tumor cells were intraportally delivered to the liver of Balb/c mice and subsequently tracked by intravital fluorescence microscopy. Coinjection of fluorescent beads and in vivo propidium iodide staining allowed examination of initial tumor cell arrest, extravasation, viability and proliferation. Results showed that GFP-transfection compared to conventional labeling procedures (Calcein, cytoplasmic microspheres) did not alter early metastatic properties. However, the long-term development of liver metastases expressing GFP was markedly reduced compared to wild type CT-26 tumor cells. An increase in the size and the number of liver metastases in T- and B-cell-deficient SCID mice suggested an immune response to the GFP transfected cells responsible for the reduced metastatic growth in wild-type mice. Based on our findings, this model can be used to examine the early steps of metastasis in vivo. However, in immunocompetent mice, the use of GFP-labeled tumor cells should be limited to tracking cell arrest and extravasation, whereas evaluations of long-term metastatic growth should be performed in immunodeficient mice.