We report here in vivo visualization of cancer cell interaction using stable sublines of green fluorescent protein (GFP)- and red fluorescent protein (RFP)-expressing HT-1080 human fibrosarcoma cells. These color-coded fibrosarcoma cell lines show similar cell proliferation and lung metastasis potential. The color-coded cells were mixed at a ratio of 1:1 and were injected into the tail vein of severe combined immunodeficient (SCID) mice. The resulting experimental lung metastases were simultaneously imaged and their ratio was determined by color-pixel analysis. Fluorescence color coding of cancer cells enables visualization of the interaction of cancer cells and can be used to distinguish cancer cells of any genotype or phenotype.