The effects of a variety of widely used anti-inflammatory agents (dexamethasone, indomethacin, and montelu-kast) as well as ubiquitous mediators of inflammation (prostaglandin E 2and nitric oxide) on the development of murine eosinophils ex vivo and in vivo have been studied over the last decade. The results indicate that developing eosinophils differ markedly in their responses to these agents from the mature forms of the same lineage, studied either in allergic human subjects or experimental animal models of allergic disease. Most strikingly, glucocorticoids strongly enhance eosi-nophil development, both in vitroand in vivo. The enhancing effects are also observed during stress reactions and are strictly dependent on stress-induced glucocorticoid hormone production from the adrenal glands. Some, but not all, of the developmental effects of glucocorticoids on eosinophils could be accounted for their ability to prevent generation of nitric oxide through inducible NO synthase, which leads to apoptosis through the CD95-CD95L pathway. A novel mechanism for the effects of indomethacin in upregulating the development of eosinophils has also been documented. Evidence that lineage-specific as well as stage-specific cellular response programmes determine these different outcomes is discussed, along with the perspectives for future research.