Tumors are infiltrated by many immune cells that influence many aspects of cancer progression and outcome. Among tumor-infiltrating lymphocytes, γδ T cells play dual functions in the tumor milieu; whereas those that produce the antitumor cytokine interferon-γ are protective, their counterparts that make interleukin 17 (IL-17) support tumor growth. Here, the authors found that IL-17+ γδ T cells express very low levels of the antioxidant, glutathione, and are very sensitive to reactive oxygen species (ROS), thus revealing their Achilles' heel.