Immune-mediated antitumor activities confront a variety of tumor-mediated defense mechanisms. Here, we describe a new mechanism involving FFA that may allow breast cancer to evade immune clearance. We determined the IC50 at which unbound free fatty acids (FFA(u)) inhibit murine cytotoxic T-lymphocyte (CTL)-mediated killing to assess the physiologic relevance of this phenomenon. We found that the IC50 for unbound oleate is 125 +/- 30 nM, approximately 200-fold greater than normal plasma levels. FFA inhibition, however, may play an important role in breast cancer because we found that large quantities of FFAs are released constitutively into the media surrounding samples of human breast cancer but not normal or benign tissue. FFA(u) concentration ([FFA(u)]) increased to at least 25 nM in 20 of 22 cancer tissue samples and exceeded 100 nM in 11 patients. Media from these samples inhibited CTL-mediated killing. Extrapolation from our in vitro conditions suggests that for tumor interstitial fluid, in vivo [FFA(u)] may be 300-fold greater than we observed in vitro. Although breast cancer release of FFA may suppress effector cell antitumor activity, strategies that reduce interstitial [FFA(u)] may significantly improve antitumor immune therapies.