Video microscopy allows dynamic observation of cancer-cell activity in the microcirculation of live animals. However, observation of cancer invasion and metastasis in situ has never been successful because of the lack of a technique for labeling cancer cells. We report here our success with video microscopy of cancer-cell activity, detection of remote metastases and cancer cells endogenously generated using the green fluorescent protein (GFP) gene as a cell-labeling system in live animals. As a cell-labeling system, GFP is stable, efficient, and nontoxic, and it is passed on to subsequent generations of cells. This pilot experiment has demonstrated the feasibility of direct observation and documentation of tumor growth and tumor invasion as well as the metastatic activities of cancer cells in live animals.