We describe the potent effect of myriadenolide (Myr), a naturally occurring labdane diterpene, in promoting the production of eosinophils in cultured bone-marrow from several inbred mouse strains. This enhancing effect is lineage-selective and requires the eosinophil growth factors, Interleukin(IL)-5 or GM-CSF. Myr acts over a very low concentration range (10−10–10−14 M), if added at the beginning of the cell cultivation. Its enhancing effect increases between 24 h and 10 days of culture. We used both pharmacological and genetical tools to analyze its mechanism of action. Several lines of evidence show that the enhancing effect of Myr requires functional integrity of the 5-lipoxygenase (5-LO) pathway, and of CysLT1 receptors, which transduce the effects of cysteinyl-leukotrienes generated through this pathway. Myr also protects developing eosinophils from apoptosis induced by exogenous prostaglandin E2 (PGE2), but not by NO, indicating that it acts upstream of NO in the PGE2-initiated proapoptotic pathway which requires iNOS and CD95. Exposure to NO concentrations insufficient to induce apoptosis abolished the ability of eosinophils to respond to Myr, suggesting the involvement of a NO-sensitive cellular target. Myr has potential as a chemically defined research tool, which can be used to generate large numbers of eosinophils, thereby overcoming current limitations in the biochemical and molecular biological study of murine eosinophils, which has so far depended on complex, labor-intensive and long-term culture protocols for in vitro expansion