The p-nitrophenacyl esters of dinoprost and its 15-epimer are well resolved using high-pressure liquid chromatography. Quantitation was achieved using the internal standard technique. The specially synthesized diphenylurea ester of cholic acid was found to be a model internal standard. Graphs of peak height ratios of the prostaglandin to the internal standard were linear with respect to the amount of prostaglandin injected, with the lower detection limit of the 15-epimer being about 0.5%. Data are presented that demonstrate the usefulness of this analytical technique in determining the concentration of the 15-epimer present during studies on the kinetics of decomposition of dinoprost.