A specific HPLC assay method for lachesine chloride has been developed and used to study the stability of lachesine in eye drop formulations. The samples were run on a C8 column with methanol 60% v/v, water 40% v/v containing 0.005 M heptane sulphonic acid sodium salt as an ion-pairing agent at pH 3.5. Using infra-red spectroscopy the major breakdown product on heat sterilization of the eye drop formulations was found to be benzillic acid.