Across most cancer types, increased macrophage infiltration is associated with a worsening prognosis. This is because tumor associated macrophages (TAMs) exhibit a variety of pro-tumor effects ranging from vascular recruitment, cell proliferation, extravasation, and immune suppression. A marker of TAMs is arginase-1 expression, which converts arginine to urea and ornithine. It is thought that arginase expression reduces the amount of arginine available to local T-cells, leading to T-cell receptor dysfunction. In this abstract, we outline the synthesis and characterization of novel compound [6-13C,6-15N3]-Arginine as a hyperpolarized 13C MRS probe to interrogate arginase activity, with the potential for in vivo translation.