P roduction of prostaglandin E₂ (PGE₂) is enhanced during inflammation, and this lipid mediator can dramatically modulate immune responses. There are four receptors for PGE₂ (EP1–EP4) with unique patterns of expression and different coupling to intracellular signaling pathways. To identify the EP receptors that regulate cellular immune responses, we used mouse lines in which the genes encoding each of the four EP receptors were disrupted by gene targeting. Using the mixed lymphocyte response (MLR) as a model cellular immune response, we confirmed that PGE₂ has potent antiproliferative effects on wild-type responder cells. The absence of either the EP1 or EP3 receptors did not alter the inhibitory response to PGE₂ in the MLR. In contrast, when responder cells lacked the EP2 receptor, PGE₂ had little effect on proliferation. Modest resistance to PGE₂ was also observed in EP4^–/– responder cells. Reconstitution experiments suggest that EP2 receptors primarily inhibit the MLR through direct actions on T cells. Furthermore, PGE₂ modulates macrophage function by activating the EP4 receptor and thereby inhibiting cytokine release. Thus, PGE₂ regulates cellular immune responses through distinct EP receptors on different immune cell populations: EP2 receptors directly inhibit T cell proliferation while EP2 and EP4 receptors regulate antigen presenting cells functions.