Apoptosis, involving both CD95/CD95L interactions and their modulation by nitric oxide (NO), is central to regulation of mature eosinophil numbers. However, its role in regulating eosinophil production from bone-marrow precursors is unknown. We examined the effects of prostaglandin E 2 (PGE 2 ) and dexamethasone on eosinophil differentiation and survival in murine bone-marrow cultures, and their relationship to: NO production as well as CD95/CD95L-dependent apoptosis. Bone-marrow cultures were established with IL-5, alone or in association with PGE 2 , dexamethasone or both. PGE 2 (10 −7 M) inhibited eosinophil differentiation by selectively inducing apoptosis in developing eosinophils. Dexamethasone (10 −7 M) protected developing eosinophils from PGE 2 -induced apoptosis. Since dexamethasone prevents induction of nitric oxide synthase (NOS), we evaluated the role of NO in the effects of both PGE 2 and dexamethasone. NO donors (SNAP and SNP) down-modulated eosinophil precursor responses to IL-5. SNAP induced apoptosis through a dexamethasone-resistant mechanism. The NOS inhibitors, Nω-nitro-l-arginine and aminoguanidine, blocked the effects of PGE 2 on developing eosinophils. PGE 2 was ineffective in bone-marrow from knockout mice lacking inducible NOS. PGE 2 up-regulated CD95 and CD95L expression in developing eosinophils. Neither PGE 2 nor SNAP were effective in cultures from CD95L-deficient gld mice. These data suggest that PGE 2 induces apoptosis in developing eosinophils through inducible NOS, leading to NO-dependent activation of the CD95L/CD95 pathway, while dexamethasone antagonizes the effects of PGE 2 on the same targets.