We report the establishment of stable, high-level green fluorescent protein (GFP)-expressing cell lines in vitro that permit the detection and visualization of distant micrometastases when they are implanted orthotopically in nude mice. Chinese hamster ovary cells were transfected with the dicistronic expression vector containing the humanized GFP cDNA. A stable GFP-expressing clone was selected in 1.5 microM methotrexate in vitro and injected s.c. in nude mice. Stable high-level expression of GFP was maintained in the s.c. growing tumors. To use GFP expression for metastasis studies, fragments of s.c. growing tumor, which are comprised of GFP-expressing cells, were implanted by surgical orthotopic implantation in the ovary of nude mice. Subsequent micrometastases were detected in systemic organs and could be visualized by GFP fluorescence in the lung, liver, and other organs down to the single-cell level. With this fluorescent tool, we detected and visualized for the first time tumor cells at the microscopic level in fresh viable tissue in their normal host organ. Confocal microscopy further enabled us to study physiologically relevant patterns of invasion and micrometastasis.