15(S)-Latanoprost is an analog of latanoprost in which the hydroxyl at carbon 15 is inverted relative to latanoprost. The IC50 values for the free acid forms of latanoprost and 15(S)-latanoprost were determined to be 3.6 nM and 24 nM, respectively, in a FP receptor binding assay using the cat iris sphincter muscle. A 3 μg dose of 15(S)-latanoprost caused a 1 mmHg reduction of IOP in normotensive cynomolgus monkeys. 15(S)-Latanoprost is a potential impurity in most commercial preparations of the latanoprost bulk drug product.