L-648,548 is a new avermectin which was evaluated for the development of an animal health formulation. A stability-indicating method for the essay of 5% (w/v) L-648,548 in an animal health formulation has been developed using reversed-phase high-performance liquid chromatography (HPLC) with UV detection (245 nm). The procedure to determine L-648,548 is linear and accurate over the range 80-120% of the target concentration with a limit of quantitation of 0.2%. Validation data are presented. Also two related degradates of this compound were observed during the stability studies of the L-648,548 formulation. These degradates were determined to be the 2-epimer formed in the presence of base and the 8a-oxo degradate formed by oxidation. Identification of these compounds following direct chemical synthesis was based on mass, UV and NMR spectroscopy. The mechanistic pathways for the formation of these degradates are discussed. The 8a-oxo degradate has a modified chromophore, thus requiring a second HPLC method with detection at 280 nm that was also validated.