Abstract: The percentage of guanine (G) plus cylosinc (C) in DNApreparations after acid hydrolysis in 88% formic acid is determined by ion pair reversed phase high performance liquid chromatography.By stepwisc gradient technique, uracil, cytosine, thymine, guanine, and adeninc are separated on a column of YWG -CH(C₁₈) with 2.5mM KH₂PO₄ buffer solution (pH=5.5) containing 5mMheptane sulfonate as the eluent(1), then with 2.5mM KH₂PO₄ buffer solution (pH=5.5) containing 18% methanol as the eluent(2).The results for the percentage determination of (G+C) are in close agreement with thermal melting point and buoyant density values determined for the DNAof calf thymus and sperm.