Microparticles (MPs) are small membrane-bound vesicles that display proinflammatory and prothrombotic properties. These particles can be released by macrophages stimulated by ligands of the Toll-like receptors (TLRs) in a process that depends on nitric oxide (NO) production. Since sex hormones can modulate macrophage responses, we investigated the effects of progesterone and estradiol on macrophage particle release in vitro, comparing the responses with those induced by the glucocorticoid dexamethasone. As a model system for particle release, RAW 264.7 cells were stimulated in vitro with poly(I:C), a ligand of TLR3. Microparticles were measured by flow cytometry, while NO was measured by the Griess reaction. As the results of these studies showed, progesterone but not estradiol can block particle release by RAW264.7 cells treated with poly(I:C); dexamethasone was also active. Furthermore, while progesterone and dexamethasone inhibited NO production under the same culture conditions, neither agent blocked the production of particles stimulated by the NO donors dipropylenetriamine NONOate {(z)-1-[N-(3-aminopropyl)-N-(3-ammoniopropyl)amino] diazen-1-ium-1,2-diolate} and (z)-1-[(2-aminoethyl)-N-(2-ammonioethyl)amino] diazen-1-ium-1,2-diolate. Studies using RU486 to assess the role of hormone receptors indicated that while this agent blocked the inhibition of particle and NO production by dexamethasone, it did not affect the inhibition by progesterone. Together, these results indicate that progesterone but not estradiol can inhibit particle release by stimulated macrophages and suggest a mechanism that may contribute to the immunomodulatory effects of this sex hormone.