We have developed a way of imaging metastases in mice by use of tumour cells expressing green fluorescent protein (GFP) that can be used to examine fresh tissue, both in situ and externally. These mice present many new possibilities for research including real-time studies of tumour progression, metastasis, and drug-response evaluations. We have now also introduced the GFP gene, cloned from bioluminescent organisms, into a series of human and rodent cancer-cell lines in vitro, which stably express GFP after transplantation to rodents with metastatic cancer. Techniques were also developed for transduction of tumours by GFP in vivo. With this fluorescent tool, single cells from tumours and metastases can be imaged. GFP-expressing tumours of the colon, prostate, breast, brain, liver, lymph nodes, lung, pancreas, bone, and other organs have also been visualised externally by use of quantitative transcutaneous whole-body fluorescence imaging. GFP technology has also been used for real-time imaging and quantification of angiogenesis.