Latanoprost is an F-series PG analog which has been approved for use as an ocular hypotensive drug. PG esters and other similar derivatives act as prodrugs which are converted to the active free acid form by an esterase/amidase activity in ocular tissues. The free acid form of Latanoprost is 200 times more potent than the isopropyl ester as an FP receptor ligand tested using the recombinant human FP receptor. The Cayman Latanoprost ELISA is a sensitive detection method for measuring the free acid form of Latanoprost. It also detects the isopropyl ester form of Latanoprost, but with less sensitivity. The assay is most appropriate for use when only one of the two forms is present. Samples containing mixtures of both the isopropyl ester and free acid should be purified and the two compounds separated prior to ELISA analysis.