Breast cancer is defined as a group of malignant cells that originate in the breast and have the potential to spread to distant sites in the body, most commonly the liver, lungs, bones and brain. Many factors increase the risk of breast cancer, including but not limited to environmental factors, age, race, gender and obesity. Methylmercury (MeHg) is an environmental metalloestrogen, a type of metal that mimics the activity of estrogen. Previous studies have shown that low concentrations of MeHg cause breast cancer proliferation via the estrogen receptor (ER), but that at high concentrations it causes cell death. However, the long-term effects of MeHg exposure on breast cancer development are unknown and thus worth researching. Many studies have used xenografts in adult zebrafish (Danio rerio) to research metastatic capability. By using xenografts of mammalian breast cancer cells in zebrafish, the effects of MeHg in an in vivo system can be studied in regards to tumor progression. It is hypothesized that dietary MeHg treatment of zebrafish transplanted with human breast cancer cells will increase tumor size and the number of tumors and will result in micrometastases. In order to test this hypothesis, zebrafish from Professor Lanni’s lab in the Biochemistry Department at Wheaton College were used. First, fish were injected with fluorescently-labeled MCF-7 breast cancer cells. Fish survival and health as well as tumor growth and metastasis were measured. Once it was known that tumors could be successfully grown, an eight-week experiment was designed in which zebrafish were injected with fluorescently-labeled MCF-7 cells. Every week, fish were imaged under a fluorescent scope; after the eight-week period, fish were sacrificed and dissected for tumors. Although further troubleshooting is necessary, a protocol was successfully developed for the injection and growth of breast tumor cells in zebrafish.