Prostaglandin E2 (PGE2) was converted into prostaglandin B2 (PGB2) by alkaline treatment and quantitated by a novel and specific competitive enzyme immunoassay using an anti-PGB2 antibody and a biotin-PGB2 conjugate as a tracer. This assay was relatively specific for PGE1 and PGE2 (n-6 type); the reactivity for PGE3 (n-3 type) was below 2% of that for n-6 type, and was applicable to the quantitation of PGE2 synthesized in lipopolysaccharide-stimulated peritoneal macrophages from mice fed either a high linoleate or a high alpha-linolenate diet.