MACROPHAGES have a key role in mediating inflammatory reactions¹. They appear in large numbers in inflammed tissue, and on contact with irritant particles release comparatively large quantities of inflammatory mediators, including prosta-glandins (PGs)². Although it is known that anti-inflammatory drugs can inhibit synthesis and consequently release of PGs^2,3, it is not clear which cellular events lead to PG formation in macrophages during phagocytosis. We report here our use of various drugs to modulate PG release and phagocytosis and our attempts to correlate the drug effects on these two parameters with morphological changes of macrophages as visualised by transmission electron microscopy. We found that PG release in macrophages is concomitant with the formation of large vacuoles. This observation suggests that the formation of these vacuoles may be a crucial event for the formation and release of PGs from macrophages.