Breast malignancies often have high levels of COX-2. The COX-2 product prostaglandin E2 (PGE2) contributes to the high metastatic capacity of breast tumors. Our published data indicate that inhibiting either PGE2 production or PGE2-mediated signaling through the PGE2 receptor EP4 (1 of 4 EP expressed on the malignant cell) reduces metastasis by a mechanism that requires natural killer (NK) cells. Tumor-derived PGE2 and exogenous PGE2 are known to have direct inhibitory effects on NK cell functions, but less is known regarding which EP receptors mediate these effects. We now show that several NK functions (lysis, migration, cytokine production) are compromised in tumor-bearing mice and that tumor-produced PGE2 interferes with NK cell functions. PGE2 inhibits the potential of NK cells to migrate, exert cytotoxic effects, and secrete interferon γ. The ability of PGE2 to inhibit NK cells from tumor-bearing mice is by acting on EP2 and EP4 receptors. NK cells from tumor-bearing mice were more sensitive to inhibition by EP4 and EP2 agonists compared with endogenous NK cells from healthy mice. PGE2 was inhibitory to most NK functions of either normal or tumor-bearing mice. In contrast, there was a trend for enhanced tumor necrosis factor α production in response to PGE2 by NK cells from tumor-bearing mice. We also report that a recently described EP4 antagonist, frondoside A, inhibits breast tumor metastasis in an NK-dependent manner and protects interferon γ production by NK cells from PGE2-mediated suppression. Taken together these data show that NK functions are depressed in tumor-bearing hosts relative to normal NK cells and that PGE2 suppresses NK functions by acting on EP2 and EP4 receptors.