To determine the effects of exercise on MDSCs and T cells, A 4T1 line of breast cancer was injected into the mammary glands of female Balb/c mice. The 4T1 tumor model is representative of an aggressive and late stage breast cancer and closely models the progression of human breast cancer. The tumors also grow in the anatomically correct position in the mouse similar to how they would grow and metastasize or spread in humans. The injected mice were put into individual cages with a wheel for them to run on voluntarily. Other cancerous mice were put in cages without wheels and therefore did not exercise. Another control group of mice were not injected with cancer and did not exercise. Daily measurements of the size of the tumors were taken on the mice that were injected with the 4T1 cells to examine the growth and progression of the cancer. After four weeks the mice were euthanized in to perform various assays to test the predicted hypothesis. T Cells and MDSCs from sedentary, exercising and control mice were collected and co cultured at a ratio of 4:1 (4 t cells to 1 MDSC) to test our hypothesis using samples of mice spleen and the Sony cell sorter to produce and isolate samples of MDSCs and T Cells. The spleen was used to collect these samples because blood platelets and importantly white blood cells are stored there. T Cell Suppression Assay was used to measure the ability of T cells to proliferate or grow and divide. The T cells were labeled with CFSE which is a dye that illuminates the cells. After 4 days of incubating the resulting CFSE dilution peaks were analyzed using flow cytometry to determine the proliferation index.