17-phenyl trinor PGF2α is a metabolically stable analog of PGF2α and is a potent agonist for the FP receptor. The ethyl amide analog has been approved for use as an ocular hypotensive drug, sold under the Allergan trade name Bimatoprost. There is some evidence that unmetabolized 17-phenyl trinor PGF2α ethyl amide is a weak FP receptor agonist. 17-phenyl trinor PGF2α ethyl amide is converted by an amidase enzymatic activity in the bovine and human cornea to yield the corresponding free acid, with a conversion rate of about 40 µg/g corneal tissue/24 hours. Cayman’s 17-phenyl trinor PGF2α ELISA is a sensitive detection method for measuring both the free acid and ethyl amide forms of 17-phenyl trinor PGF2α. The assay is most appropriate for use when only one of the two forms is present. Samples containing mixtures of both the ethyl amide and free acid should be purified and the two compounds separated prior to ELISA analysis.