Ionophore antibiotics are widely used in veterinary medicine as anticoccidial drugs for poultry and as growth promoters for farm animals. The ionophores, despite their utility, possess a narrow range of safety; several species (horses in particular) appear highly susceptible to the toxic effects of these compounds, compared with other animal species. The purpose of this study was to apply a quantitative high performance thin layer chromatography (HPTLC) method to identify monensin and lasalocid residues in feeds. We then applied this method to the determination of liver residues. Our method, slightly modified from literature data, appears to be specific, linear (r2>0.99), repeatable and reproducible, sensitive (limit of detection of 0.2 mg=g in liver for monensin, 1 mg=g for lasalocid), convenient, and simple to use.