Abstract: The growth hormone（GH） standard protein and preparations were analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry(MALDI-TOF MS). The molecular weight of the intact GH was 22 124 u， and the sensitivity was 74 fmol. Another GH preparation shows that the molecular weight is 22 254 u. The difference is 131 u compared with the GH standard protein. The secondary mass spectra of the trypsin digested peptides confirms that there is a methionine residue on the N-terminal. The eight digested products are sequenced by TOF/TOF, and the recoveries are 45.5%. The protein score is 132， and protein confidence interval is 100% by Mascot protein library. The result indicates that methionine on the N-terminal can be used as the molecular marker to distinguish exogenous GH.