Fluorescent proteins are available in multiple colors and have properties such as intrinsic brightness and high quantum yield that make them optimally suited for in vivo imaging with subcellular resolution in the live mouse. In this protocol, cancer cells in live mice are labeled with green fluorescent protein (GFP) in the nucleus and red fluorescent protein (RFP) in the cytoplasm. GFP nuclear labeling is effected by linkage of GFP to histone H2B, and a retroviral vector is used for cytoplasmic labeling with RFP. Double-labeled cells are injected by various methods. High-resolution imaging systems with microscopic optics, in combination with reversible skin flaps over various organs, enable the imaging of dual-color labeled cells at the subcellular level in live animals. The double transfection and selection procedures described here take 6-8 weeks. Cancer cell trafficking, deformation, extravasation, mitosis and cell death can be imaged with clarity.