e21130 Background: Breast cancer is ranked second among the most common cancers in the world. Radiotherapy (RT) is known to reduce the risk of the recurrence in cancer treatment. Substance P (SP), binds to the Neurokinin-1 (NK1) receptor which plays a role in inflammation, proliferation, angiogenesis and migration in cancer cells. MIP-2 (Macrophage inflammatory protein 2, IL-8 in human which has been currently understood to play a role in carcinogenesis is an inflammatory CXC chemokine. The aim of this study is to show whether MIP-2 levels alter with RT and whether SP has a role in this alteration. Methods: In this study, we investigated the effects of SP, NK1 receptor antagonist (NK1 RA) and RT on cell proliferation and MIP-2 secretion in 4T1 cells. 4T1 cells were derived from spontaneously formed breast tumors in Balb-c mice. We have cultured its subsets from metastatic lesions, and we named them as 4THM (heart metastasis), 4TLM (liver metastasis), 4TBM (brain metastasis). Cells were plated as 400-500 cells per well in a 96 well plate containing medium with 5% FBS. After 24-48 hours, cells were applied NK1RA and SP. After 3-4 hours of treatment, the group of RT was irradiated with 45 Gy. The proliferation of the cells was evaluated 48 hours after treatment. This proliferation was evaluated using WST cell proliferation reagent (WST-1, Roche). MIP-2 levels were measured with ELISA. Results: We observed that RT reduced the proliferation of cells in a ratio of 30,2 % in 4T1 cells, 43,9 % in 4THM cells, 44,7 % in 4TLM cells and 34 % in 4TBM cells. 4THM, 4TBM cells have the highest MIP-2 secretion, which was further increased after RT. Also, it was observed that SP, 10-8 M concentration decreased the RT-induced MIP-2 secretion. Conclusions: Increasing MIP-2 secretiondue to RT can precipitate cancer development and SP can be used as a therapeutic aim to prevent such an increase.