Phospholipase A(2) (PLA(2)) comprises a superfamily of enzymes that hydrolyse the ester bond of phospholipids at the sn-2 position(1-3). Among the members of this superfamily, cytosolic PLA(2) has attracted attention because it preferentially hydrolyses arachidonoyl phospholipids and is activated by submicromolar concentrations of Ca2+ ions and by phosphorylation by mitogen-activated protein kinases (MAP kinases)(4-8). Here we investigate the function of cytosolic PLA(2) in vivo by using homologous recombination to generate mice deficient in this enzyme. These mice showed a marked decrease in their production of eicosanoids and platelet-activating factor in peritoneal macrophages. Their ovalbumin-induced anaphylactic responses were significantly reduced, as was their bronchial reactivity to methacholine. Female mutant mice failed to deliver offspring, but these could be rescued by administration of a progesterone-receptor antagonist to the mother at term. Considered together with previous findings(9-15), our results indicate that cytosolic PLA(2) plays a non-redundant role in allergic responses and reproductive physiology.