Abstract Prostaglandin-E 2 9-ketoreductase (PGE 2-9-KR, EC 184.108.40.206), the enzyme which catalyzes the reaction from prostaglandin E 2 (PGE 2) to prostaglandin F 2α (PGF 2α), was purified 580-fold from swine kidney. The molecular mass of the enzyme determined by SDS-gel electrophoresis was 33 kDa. Antiserum against the purified enzyme was raised in three rabbits. The antiserum was able to precipitate PGE 2-9-KR from swine kidney and to crossreact with PGE 2-9-KR from several reproductive organ tissues, such as rabbit ovary, rabbit corpus luteum, rabbit endometrium and human decidua vera. When swine kidney PGE 2-9-KR was labelled with 125I and incubated with affinity-purified antiserum in the presence of increasing amounts of unlabelled enzyme, competitive binding of the unlabelled enzyme to the antibody was observed. A radioimmunoassay for the quantitation of the enzyme was developed. The standard curve was linear from 5 to 500 ng enzyme. The intra- and interassay coefficients of variation were 6.4 and 13.2%, respectively. The assay may be useful for the quantitation of PGE 2-9-KR in several tissues under various physiological conditions.