A reversed-phase high performance liquid chromatography (RP-HPLC) method for the determination of insulin potency in preparations has been used. The method takes a unique standard solution (40iu/ml in 0.6mg/ml EDTA) which is stable enough to permit use for >1 month. NPH insulin suspensions are clarified by addition of heparin, lente insulins are clarified by addition of tetrasodium EDTA, and EDTA is added to neutral regular insulins. Chromatography was done using a 0.1M pH 2.0 phosphate/25%CH3CN mobile phase and reversed-phase Zorbax TMS column. The HPLC assay results was approved to agree with the bioassay method.