Casein-elicited peritoneal exudate cells (PEC), mainly consisted of neutrophils, were collected from platelet-activating factor receptor-knock-out (PAFR-KO), cytosolic phospholipase A₂ knock-out (cPLA₂-KO), and wild-type (WT) mice. After stimulation of PEC with calcium ionophore A 23187, PAF levels were measured by radio-ligand binding assay using receptor-rich membrane fraction prepared from the PAF receptor transgenic mice. We found that the level of PAF production by PEC was not different between WT and PAFR-KO mice. On the other hand, cPLA₂-KO mice were deficient in the PAF production. These results provide the direct evidence while cPLA₂ is essential in the production of PAF, PAF receptor deficiency has little effect on the PAF production.