The primary structure of rhFK506-Binding Protein (FKBP) 12 was studied with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) by measuring its molecular weight and tryptic peptide mass fingerprinting.The calculated molecular weight of rhFKBP12 was 11819.5 and the measured value was 11818.0. The relative error was only 0.01%. The measured peptide mass fingrprinting of rhFKBP12was identical with that deduced from its amino acid sequence. It proved that the primary structure of rhFKBP12 was correct and there was no amino acid deletion, mutation and modification in its expression, refolding andpurification.